macOS stores the folder icon is some weird hidden file inside the folder called Icon\r That is, the filename is Icon and then the carriage return character.
When syncing with Windows, this is an invalid character so syncing constantly fails, Syncthing on Windows reporting the folder as Out of Sync.
I have added the Icon\r exception in .stignore, but the global state still retains the file and Windows still complains.
I don’t know how control characters are actually represented, but if it’s literally you need to escape the backslash in the pattern, i.e. use Icon\\r (or *\\r to catch all items with carriage returns at their ends).
To get it out of the global state, you need to remove it from the .stignore, remove the actual file and ad it to the .stignore. Now you can create the file / add the folder icon again.