After starting the synchronization, the dialog closes and a QGIS project is reloaded several times (offline/original project). Until the end of (successful!) Synchronization can take up to 15 minutes for a large project (even though only one feature in a layer has been deleted). However, there is no progress display in the “Synchronize from Qfield” dialog (although progress bars are visible there, but the dialog closes immediately after start) nor is there a completion message via QGIS message.

That is completely unsatisfactory. You can't work on the project during synchronization anyway, so why doesn't the dialog with progressbars remain modally visible? You never really know whether the sync is done; only because of the blocked UI thread, which then freezes all of QGIS if you click somewhere in the main window prematurely.