Wow, they responded super quick this time - about 22 minutes ago the ticket was answered - here’s the final response, which I’ll go over below;
Greetings,
The issue with the cache giving the incorrect item levels happened and was identified shortly after launch on Tuesday, April 23. The timeframe used was from launch to when the WoW team placed the hotfix. In this case, anyone who opened their cache prior to 2:30pm PDT on April 23, received a new cache. In your case, you opened your cache on 4/24 at 10:33am PDT.
Please note that the fix in this case was to provide players who opened the cache during this time period noted above with a new cache. Unfortunately, the curio issue was a separate issue occurring around the same time.
Unfortunately, the fix implemented could not easily identify players that experienced the issue with the curio and could not be provided across the board to all impacted players fairly. I certainly understand that this isn’t ideal in your situation; however, the fix will not apply to you or others that received the curio after the inital issue was resolved.
Blizzard Customer Support is unable to directly assist with bug assistance, and compensation for these issues will be handled by the WoW game team. Unfortunately, when bugs happen, compensation is not always possible. As in this case, the game team will do their best to correct bugs and issues that arise from them,
In some cases, there will be impacts that cannot be effectively fixed for all players, like this case for anyone that received the curio. The CS team is unable to provide details of how fixes work or why some issues are harder to resolve than others.
As always, when a bug occurs, please report it in-game (https://battle.net/support/article/000339262) via the Bug Reporting option, so the game teams can be made aware of it and they can 1) Fix the issue and 2) Determine if/how any compensation is possible for impacted players.
I would also suggest posting on the bug report forums as well, so other players can share specifics that could help the game team and QA identify and resolve the root issue.
The response on the CS side will not change for this issue. There’s no additional information or assistance we can provide for this issue.
Again, I know this isn’t the answer you’d like to receive, and I apologize that you and others ran into this situation.
Jornairr
Blizzard Entertainment
To begin;
anyone who opened their cache prior to 2:30pm PDT on April 23, received a new cache. In your case, you opened your cache on 4/24 at 10:33am PDT."
So, as long as you’re a US player (their maintenance day is Tuesday, not Wednesday like Europe servers) and opened it before 14:30 PDT (10:30pm UK time - PDT is -8 hours behind the UK), you were considered an ‘affected player’ and received the cache.
To note, 10:33am PDT would have been about 6:33pm UK time for me, on the 24th.
I find this a little unreasonable, and slightly odd, considering the issue really shouldn’t have affected ANYONE on Europe servers if they had actually fixed this bug, prior to the Wednesday maintenance, and even more so being so late on in the day for me, as they should have included the bug fix in the rollout of the update to the Europe servers… that’d make sense, right? Apparently, not to Blizzard.
Next point;
Please note that the fix in this case was to provide players who opened the cache during this time period noted above with a new cache. Unfortunately, the curio issue was a separate issue occurring around the same time.
Alright, so… if this is an entiiiirely separate issue… has it been addressed? Where is the compensation for this? It’s pretty much the same thing. The ilevel for the curio (441) was still below the ilevel of the cache (point and issue number 1), on top of that, the item is - as a response earlier states - completely unusable/redeemable in the current season, as I believe the vendor has been updated/removed to no longer accept it, in it’s current form at least.
So why was this not addressed? Again, where is our ‘replacement item’ for something no longer usable, for the timescale until it was fixed? I didn’t see ANY update on the issue being resolved with the cache, let alone anything else, until long after I had opened the cache to begin with, which is odd huh?
Last thing;
Unfortunately, the fix implemented could not easily identify players that experienced the issue with the curio and could not be provided across the board to all impacted players fairly. I certainly understand that this isn’t ideal in your situation; however, the fix will not apply to you or others that received the curio after the inital issue was resolved.
I get I’ve covered some of this already, but I wanted to note - they were easily (and quite quickly, I might add) able to establish when the fix was put in and when I, specifically on my Monk, opened the cache. So they have access to whatever logs they need and are quickly able to see loot picked up from nearly a week ago, without much effort. Are people within Blizzard that incapable they’re not able to setup and run a script or something similar, to parse data and gather this data in bulk? They can do ban waves for bots, but not this? 
Guess I’ll close the ticket at this point, as they’re adamant that I, or anyone else for that matter, are not getting a damn thing from them, because we’re just poor EU plebs that are not able to time travel, but instead throw money at a company that seemingly does no testing or Q&A prior to updates after so long of having repeated issues pretty much every patch/update day and apparently cares not about it’s paying customers. Sad times. 
that’s why i wasnt doing TW at the time when I heard of the news from US servers