Definitely has been that way in Word. IIRC the “open - close - contents-changed-while-mtime-stays” dance was mostly Excel and Word.
Rather off-topic indeed if an application other than Syncthing is changing the hashable contents of a file. 
A good reason to dislike such propritary software, and also a reason to love Syncthing: the latter provides an easy-for-beginners way to acheive a content-versioning system.
In other words, Syncthing protects users by preservering the unaltered file when faced with such foolish questions as “Save changes?” when there are no changes. 
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