As part of installing the add-on, I did read the FAQ and set things up correctly, including having the bot at the top of role hierarchy. The synching worked as expected yesterday when initially set up and for all users who connected their Discord account last night. With no changes to any of the settings -- in the forum or Discord -- all users who had been previously granted a role yesterday no longer had the role today. I had another user associate their account after I discovered the issue, and not only did they not get bumped to the moderator role, they didn't even get the default role I set for registered members.
Initial thought was that the Discord server might not have been communicating, but connection tests were successful. Re-running the cron entry didn't resolve it, nor did disassociating and reassociating individuals with Discord. I was finally able to find a (sort-of) resolution by 'editing' users. I made no changes, just pulled up the user in AdminCP and hit Save, then within seconds, Discord showed them with the role they should have had. Did the same to one of the users who lost their role since last night, same result. Then I did a batch update to all 'Registered' users to add 'Registered' as a secondary permission group. All associated accounts were then synched.
Hoping this was a one-time issue, and if not, I hope this workaround at least helps someone else.
Even while accounts were not being synched, however, the server bot was posting threads and replies without interruption.