BTRFS is smart enough to check for file errors in some situations under normal operations (I forget which, it’s been a while). When it finds issues, it puts it in read only to try and prevent things from going off of potentially corrupt data.

NTFS, which is what Windows usually uses, is a very “dumb” file system. It is merely a record of what files exist where, so if data corrupts, it will only throw NTFS off if it’s in the file index it stores itself. If it’s in the middle of your file, NTFS doesn’t know and doesn’t care and will just give you the wonky data.

Windows seemingly continuing to work is just a consequence of the “dumb” file system. It will take some critical file getting corrupted before Windows or some program will just crash. At least BTRFS is trying to tell you things are looking amiss and you should definitely back up anything important.