If you have a variable that is set to a number and you sync it, any values added to it will be added to the end.
If the variable value is 3, it goes like this:
Before syncing: 3 + 1 = 4
After syncing: 3 + 1 = 31
This is what happens on jeje52-test3, where I found the bug. I hope I'm not just doing it wrong.
Edit:
I would like to see this bug get fixed, I reported this more than a month ago.
I think the variable value type turns to string when synced, just change that to integer if it's only a number and it's fixed. But I don't know much about how the game works so I could also be completely wrong.