zvainer Thank you for detailed report.
First, we need to separate here what is google drive issue, and what is Tonfotos issue.
As you can see on the second screenshot, images that did not get thier tags synced, have later modification date than images with tag. And based on the information you provided, it is not clear, were there indeed some modifiction to those files on second PC (with or withour Tonfotos), or this is just google drive bug.
If someone or something modified those files, then google drive behaviour was totally correct - it replaced files with more recent one. As a result, I beleve, files on first PC should also lose tags.
Please investigate what is happening with those files, who or what is modifing them. This is the core to understand why sycnhronization is not working as expected.
The second issue is why Tonfotos on he second PC does not detect that file has been changed and does not read its tags. This again has to do with modification dates. Tonfotos relies on modification date from the files system to detect changes. If modification date has not changed since last time, program will not spend time reading this file once again and analyzing its metadata. Can you please check what is happening with modification date when you modify tags? Does it change? Does it change on both PCs?
