I have just Today, 7th July 202, updated to the Beta version 18.104.22.168.
I have .mxf files with 8 mono tracks of audio, after modifying channels to select trks 1 and 2 only, as a single stereo pair, Premiere immediately crashes once I select OK. I have tried this with several different files, all with the same result.
Thank you for the report! I have reproduced this issue (don't even need to make changes, just click OK button in any of the Modify dialogs will repro crash) and will log a bug for our Devs to investigate.
This is being tracked under bug DVAPR-4224771
This should now be fixed in Premiere Pro 14.3.2 beta build 023+
This is happening to me since upgrading to 14.6.0