Perhaps this is the relevant upstreams bug report:
https://bugzilla.gnome.org/show_bug.cgi?id=789491
("Bug 789491 - mtp volume is not removed when unplugging")
If so, it was supposedly fixed in gvfs 1.35.2, "mtp: Fix volume removal
with current udev behavior":
https://git.gnome.org/browse/gvfs/tree/NEWS
Torbjörn Andersson