Thank you for including your version number! The frame rate problem is unique to that version. The bundled version of FFmpeg was updated which meant a change to API calls, but the MP4/MOV muxer wasn’t ready for prime time yet. Those issues were resolved in the October release. In theory, you could open your projects in the October version and export again to get constant frame rate files without any additional editing or configuration work.
Here are the release notes, with the fix being item 1:
And here is someone else reporting the same problem if you want more details: