Generally, before I try a beta version of Joplin, I'll add a firewall rule to block its Internet access first so I don't have to worry about any adverse effects to the cloud backup if I want to revert to the latest stable.
It occurred to me that maybe I should submit a feature request for a launch parameter that disables syncing within that process. If launched again without the parameter, syncing would work as before.
The Synchronize button's label could be grayed out with 'Sync disabled' as the label, and the options within the Synchronization tab of the options will explain in big bold letters at the top that the software was launched with the no-sync parameter and for synchronization to occur, it needs to be launched without that parameter. The 'Sync wizard' and 'Re-upload local data to sync target' buttons would be grayed out, but other options will allow changes in case the program was launched specifically to remove the Synchronization target or to delete local data and re-download from the sync target without the automated sync occurring.