Hello Ton,
As a quick recap for anyone else that may be experiencing this issue: Some appointments were not synced because of a URL decoding issue when comparing calendar UIDs sent from Google. This bug has been resolved and will be included in the 16.x minor update that's scheduled for tomorrow.
Thank you for helping us to improve SmarterMail!
Andrea Free
SmarterTools Inc.
877-357-6278
www.smartertools.com