exteding a clip once it's been cut

This is now fixed as of Premiere Pro (Beta) 26.1.0 x004.

Thanks @Mark22576097bsvw for confirming.  I'll update once this is fixed in a build.

Yes, I have it set up on external monitor and I'm on Mac. That works now. Thanks!

Hi @Mark22576097bsvw - Thanks for the report.  
Do you have Transmit enabled and set to an external monitor in Prefs>Playback and is "Composite Preview During Trim" enabled in the timeline wrench menu? If so, there is a bug in Beta if you're on mac. The fix for this bug is currently in development. Please confirm if this is the issue that you are experiencing.  As a temporary workaround, you could disable Transmit or "Composite Preview During Trim".  Thanks!