(Based on issue mentioned here)

STR: Run attached test case
Expected: "Hi" should dis/appear every second
Actual: "Hi" never appears

Just in case: I did play a bit with the minimal case, and found out that the minimal workaround could be to have a selector that matches the same element with the same :has() but without the pseudo: just by having it, the other selector with the pseudo starts to work: https://codepen.io/kizu/pen/wvNwWGo

Also, was a bit tricky to debug on CodePen, as it seems that sometimes (not always?) editing the styles pane did trigger something, making the case with the pseudo-element to work.

(In reply to Roman Komarov from comment #1)

Just in case: I did play a bit with the minimal case, and found out that the minimal workaround could be to have a selector that matches the same element with the same :has() but without the pseudo: just by having it, the other selector with the pseudo starts to work: https://codepen.io/kizu/pen/wvNwWGo
Also, was a bit tricky to debug on CodePen, as it seems that sometimes (not always?) editing the styles pane did trigger something, making the case with the pseudo-element to work.

(A bit late, but...)
Yeah, if any other selector happens to trigger invalidation on the right element (i.e. the "parent" of the pseudoelement), it look as if it works correctly, but the underlying problem is not being able to invalidate on its own.
From the author's perspective, I agree that it just seems inconsistent and frustrating. At least the silver lining is that this in and of itself tends to be a telltale sign of this type of bug.

https://hg.mozilla.org/mozilla-central/rev/d02d47db8d23

Reproduced on an affected Nightly build from 2023-10-19, on Win 11.
Verified as fixed on Firefox 121.0b3 (20231124091512) and 122.0a1 (20231126211752)on Win 10, Ubuntu 22 and macOS 11.