In my css i had set H2 font-size to 20 px, in the accordion css I have 18px.
The accordion interface lets you choose title size, which is in my case set to H2. But for some reason, accordion would not accept this setting. You have to exactly mach your H2 settings from your basic typography settings.
I also found, that if you comment out certain settings in your accordion css, then generic bootstrap5 values apply.
If that is a clear Concrete issue I cannot tell.
Anyway, accordions behave weird and need tinkering to adjust.