moving files within MTP mounts creates duplicates
When a mobile device (tested: Huawei P40 phone) is accessed through MTP, attempting to move a file from one directory to another (both on mobile device) seems to instead result in creating a duplicate copy of the file, leaving the original in the same place.
This appears to be a bug in Thunar, rather than GVFS, because the Caja file manager from the MATE desktop system performs the same operation correctly, without creating duplicates.
Conversely, using Thunar to move files between directories on a regular filesystem mount, does not create duplicates. The duplication bug appears to be specific to the GVFS or MTP code within Thunar.