Cache tags sometimes don't invalidate properly #59740

Hello.
We've noticed some issues with cache on our application. We update currencies (global) once a day, and then store them in cache for later use. All the important places we absolutely need to refresh have a shared tag added to their cache (through symfony's TagAwareCacheAdapter using RedisTagAwareAdapter). But we noticed that sometimes invalidating tags doesn't work as expected. It seems to work fine in tests and on the dev platform, but not the production one (sometimes, again).

We have over 540k cache keys (on the lower end), say about half of them may contain that cache key. Redis doesn't seem to be the bottleneck, as far as we can tell.

Additionally, can cache tags contain dots? It doesn't error, but I wonder if it's a problem or not. The page on docs about invalidation doesn't mention it, so.