@dwg, I think you have expressed the issue quite nicely and this is probably the root of the problem. Having two systems synchronize but have incompatible actions in this case auto committing two different transactions that should be a single transaction is the problem. I am not sure how the software would logically solve this issue other than asking the user for guidance.
The following rev notes are included for 2019 beta (1824)
- Delay auto-committing of reminders until after the first sync pass is complete - Added preference setting for whether auto-committing reminders should be processed for the current file on the current compute
You should not replace your 2017 build until stable release.