In certain apps (firefox, chromium, calibre) which I believe are GTK-based apps
the font size keeps switching to very tiny and then back to normal

Trying to work out the sequence is not easy but here's what I found.
I run daily updates and so far this week I've had:-
   Sunday - Ok
   Monday - normal sized fonts
   Tuesday - tiny fonts
   Wednesday - normal sized fonts
   Thursday - tiny fonts
   Friday - normal sized fonts
   Saturday - tiny fonts

As far as I'm aware I've changed no system settings myself (while searching for
a resolution) but it seems to change font size in synchronisation with updates.

Operating System: openSUSE Tumbleweed 20230217
KDE Plasma Version: 5.27.0
KDE Frameworks Version: 5.103.0
Qt Version: 5.15.8
Kernel Version: 6.1.12-1-default (64-bit)
Graphics Platform: X11
Processors: 12 ��� AMD Ryzen 5 1600 Six-Core Processor
Memory: 15.6 GiB of RAM
Graphics Processor: NVIDIA GeForce GTX 970/PCIe/SSE2