This is the timeline:
Last reset (Thursday I think), I did a Mythic 0 of Ruby Life Pools. I received a +2 key for Waycrest Manor.
Last reset (Friday I think), I did a Mythic 0 of Brackenhide Hollow.
Last reset (Tuesday), I used my Waycrest Manor +2 key. It got depleted.
This reset (Wednesday) I opened my vault and got a cloak. I didn’t receive a new M+ key, but I figured that’s because I didn’t actually finish an M+ dungeon in the last reset.
This reset (Thursday), I did a Mythic 0 of Ruby Life Pools, but I didn’t receive any M+ key.
I went to Lindormi in both Valdrakken and in the portal room, but she doesn’t offer me any key. The only dialogue options I have with her are “Where can I find [dungeon]?”
I looked around on the forums, Reddit, Wowhead and other places to see if this is a known bug or something. I saw there were some reports 1-2 years ago, but they could all go to Lindormi and just ask for a new key.