I really hope it is not the case, because that would be a first step to abandon Mylio.
It is also a fact, that the GPS data was not updated, but the Title was, even when an XMP sidecar was there for a JPG file, but the Title was empty.
I might make another try with overwriting other fields and see, how Mylio behaves there.
Then at least it would need an option what data to prefer: the one with the latest timestamp or the one in the database.
Should this be really by design, I would let Mylio save the metadata to all my phone files right away, but this would cause a myriad of duplicates, as Mylio is not able to tell, if a file is already imported (by Mylio) from the phone, if one saves metadata to the same file.