From 1:00 you can see the popup missing texture, using the Padoka Unstable
https://youtu.be/dCxsWEr2pOw?t=61

Note that this problem is more frequent on Mesa shipped with Padoka Stable and
Oibaf PPAs, and nothing on the 18.0.5 shipped as default from Canonical.

I wasn't able to reproduce this issue on Firewatch with Padoka Unstable in a 5
minute gameplay, but i have experienced the same with the Stable on that game,
and nothing with the Canonical version.