I’ll make two suggestions:

Update your hamlib version. I compiled hamlib for Mac this morning at:

https://www.dropbox.com/s/uf9j4km8oliiz48/wsjtx.260rc5.zip?dl=0

In Finder, go Applications, right click on WSJT-X app and select show package contents, click on contents and paste the file into the preferences folder. Tell it yes when it asks whether to replace your old file.

There was a problem with the hamlib that was shipped with rc5 that impacted Flex radios that was corrected in the current hamlib.

I’ve also seen it suggested that you rename your WSJT-X.ini file found in ~/library/preferences. WSJT-X will create a new one but you’ll have to reenter all your preferences in the general, radio, audio tabs etc in WSJT-X.

If these don’t help, I’m fresh out of ideas.