Umutcan Hi, I am a professional SEO. If you share at least one affected URL we might be able to help you. There are several things that could be going on here. So make sure to:
The URL is in a guest accessible category. This way GoogleBot won't have any problems reading it.
The URL is not blocked by robots.txt or any parameters (?,=...etc).
Make sure you have updated Flarum SEO and BA extensions.
You will need to rule out that the problem is not caused by another extension. So disable all extensions and leave only Best Answer enabled. You can then turn extensions on and test an affected URL with Google's rich data testing tool.
If you see something like this in the rich data tool, then everything is fine for you, sometimes Search Console warns about errors, but then they don't affect the SERPS.
I can confirm that the schema markup of the extension works correctly.
Sorry if my English is not correct, it is not my main language.