DM does not change the block device out from underneath a mounted filesystem. The filesystem (and processes that are using it) would not be able to reliably cope with such an abrupt change to its underlying block device.
The user must decide to merge a snapshot. Once that decision is made the user will initiate the merge using lvm2's lvconvert --merge $VG/$LV
But the actual merge cannot start until both the snapshot that will be merged and its origin device are closed.
So if either the snapshot or origin device are still open (e.g. the filesystem ontop is still mounted) the merge is deferred until the origin LV is next activated.