Example Project UIThreadUpdate Project Bar Stops

The example project /Applications/Xojo 2020 Release 2.1/Example Projects/Desktop/UpdatingUIFromThread/UIThreadUpdate.xojo_binary_project appears to work fine on a raspberry pi. Click the Start button and the progress bar runs.

However, if I add a second button that shows a message box:


The progress bar stops advancing when the message box is shown. Close the message box and the progress bar starts advancing again.

On MacOS the progress bar continues to advance when the message box is shown.

Is this a known bug and is there a workaround?