Browser: Latest (UXP-based) Serpent 52.9.0 (2022-02-25) (32-bit)
Browser profile: Fresh/clean one (with default settings), only this extension installed
Extension version: Latest release, i.e. v1.12.15

STR

1. Make sure you're already signed-in, as this GitHub feature is reserved for GitHub members...
2. Load e.g. https://github.com/SeaHOH/github-wc-polyfill/releases
   In the "reactions" area, just below "Assets", you'll see that (at the time of this writing) 6 members have reacted with 👍 and another with a ❤️ emoji.
3. Place the cursor on top of either reaction;
   expected behaviour: A tooltip containing the usernames of the members that reacted similarly:

(the above was taken with a Chromium-derived browser)

actual behaviour: No tooltip is being displayed:

(St52+gh-wc-pf-1.12.15)

NB: "Reaction" tooltips are being displayed correctly on Comment reactions...

Addendum:

It later emerged that the GitHub editor's toolbar buttons are similarly affected, i.e.

1. Scroll down this page to find the "New Comment" input field (you also need to be signed-in).
2. Hover with the cursor over one of the editor's toolbar buttons,

expected behaviour: A tooltip pops-up, explaining the function of selected button:

(Chromium-derived browser)

actual behaviour: Tooltip missing:

(St52+gh-wc-pf-1.12.15)