Thunderbird Calendar syncing issue

I am using TB portable v 60.8.0. I do a lot of traveling and have TB on a USB stick. We have 5 people in the office and everyone has their own calendar using TB and I can see all 5 calendars on my machine. All off the calendars are google calendars.

What I am finding is that if I don't syncronize my USB every week or so, when I do syncronize, TB acts as if no calendar entries were ever on the calendar and goes and syncronizes everything back to the beginning. I then have to wait until 50-60,000 entries are syncronized before I can start using TB. In account settings under synchronization and storage I am not downloading emails but I don't have the issue with emails, only the calendar.

I hope this is clear. If I plug in the USB drive every day, no problem. It's only when it's not syncronized for a while (maybe 1-2 weeks) that it goes and then seems to resyncronize everything. Even after not syncronizing for a while, if I start the calendar offline, all of the expected calendar entries are present so I don't know why it keeps resyncronizing old entries it already has.

Thanks for any help you can give me. Hopefully it's just a settings issue I have overlooked.