cache_messages table empty even though message caching is active

[smares]

Hello, can anyone give me some hints why the cache_messages table could be empty even though Roundcube is configured to cache messages? The TTL and threshold are left as default (I believe it's 10d and 50 KB). I tried opening all kinds of mails, small ones with only a few bytes, and larger ones with a few KB.

Edit: Oh, forgot to mention that I obviously use 'db' caching.