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!