Can’t pinpoint when it started happening, but, I am having issues when healing using @mouseover on default Blizz Raid frames;
I could be stood directly next to my target , and when using a @mouseover heal, its claims my target is “out of range”. But if I mouseover their character directly it works. It doesn’t happen to all friendly targets making it difficult to isolate the issue.
Its worth noting that this has been happening in normal BGs.
Also to note, I have removed any “invisible” bars that might have covered my UI Raid frames, so this isn’t the cause.
I need to fix this, or this needs to be fixed as a bug as it’s game breaking.
Initial suggestions - have you tried disabling all addons temporarily and seeing if the problem persists ? I assume you’re using mouseover macros and not an addon that does the heavy lifting for you…
If you’re using an addon for mouseovers it could be that. You could try writing a simple macro in-game and using that to see if the problem goes away using the macro.
Here’s the one my priest uses for ‘flash heal’ - you can adapt it for a paladin 
#showtooltip Flash Heal
/cast [@mouseover,exists] Flash Heal
Yeah I use @mouseover macro like yours except I have [help] in there too. I removed that thinking it might bug with cross faction but you can’t have cross faction in normal BGs.
Played again today and @mouseover macro gave me the same “out of range” error when over my own target in the Raid Frames! It works on my player icon. Also works on enemy target’s target. It is literally just the Raid Frames.
Additionally, I noticed that someone left the BG, only to be replaced by someone else who ended up sharing the same box in Raid Frames!! The writing overlapped making them unreadable! Again, this is all Blizzard default UI Raid Frames.
Will test by turning off other add ons later when I get chance to test. Don’t really want to get another add on just to provide me Raid Frames because default Blizzard one is botched.
I wonder if, somehow, it’s the [help] bit. In a way it’s redundant - spells you cast on friendly targets cannot (usually) be cast on unfriendly targets. The way it works with my macros is that if I point at an entity that I cannot heal (thinking of my priest here) I receive the heal myself. The same sort of principle would apply with a damaging spell; you cannot cast it on a friendly target. That’s why I think [help] is mostly redundant and [exists] is enough. I’m now waiting for someone to wade in with an exception I haven’t considered
I would try substituting the macro I use for one of yours as a test to see what happens.