Yes. But it doesn't really bother me.

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Steam Play games tend to do that. Don't know why. Even if Proton updates there should be no reason to update the game itself.

I seem to remember it happened before with Native Linux games for a short while till they found & killed the bug causing it.

I don't know guys if you're aware of issues tracker Valve has on Proton github page, but if you aren't I strongly recommend to visit it. In general they track:
- every issue with whitelisted games,
- one global "issue" per not-whitelisted game to track all the issues in this game
- general issues that are not related to particular games.

The issue you just mentioned is well known however it is not addressed yet (Please visit referenced ticket as well). Please also get familiar with this issue, as it describes, that sometimes updates are 354.3 MB, as it means that Proton is downloaded (which is little bit weird for me, as AFAIK there is no per-game Proton instance).

Anyway, I visit Valve's tracker everyday, of course I do not read all the issues, but I learn which games are problematic, which games run flowlessly (of course you can get this information from google spreadsheet / proton city), what are the general issues etc.
I also add my compatibility reports (and I recommend you do it as well, as it provides important input for Valve to know about issues Proton users have and let them focus on particular issues).

I've taken to having three pages open when I'm looking up games to test.

Steam Play Compatibility Reports

Github Valve Proton Issues

WineHQ App Database

That way I can see if a game has been tested & what resulted for others & also submit a report to the compatibility site if needed.

I'll have to sign up to Github as well now so I can report things there.

It's just shader cache.

Sometimes updated games are getting corrupted and you have to delete the whole game and reinstall it again in order to resolve this issue.