I have done some additional digging:
My guess is, that this commit in the discourse core is what caused the break:
https://github.com/discourse/discourse/commit/1ea6dd56caccc051eda26e15f5eb0d34fa5a5566
Specifically this:
seems to interact badly with this styling from the theme component:
Deactivating this css line in the dev tool doesn’t change the position of the currently displayed reactions picker, but causes the next one to be placed correctly.