Reassigning Calendars and Tasks in Thunderbird

I am able to add Mailbox calendars and tasks into Thunderbird Desktop using using my App Password credentials. For the most part, they sync fine.

However, within Thunderbird, I am unable to reassign a calendar event from one calendar to another. I'm also unable to reassign a task from one list to another. Doing some looking online, the only solution I could find was forcing TB into offline mode, then making the change, then going back online, and then syncing.

Does anyone know why this is happening and how to fix it so that going offline each time I want to make a change isn't required? Disabling offline support for individual calendars/tasks did not fix it.

I'm unsure if this is a Mailbox.org issue, a Thunderbird issue, or some combination of the two. I'm on beta for what it's worth.