I have observed as an end-user that the mtp doesn't work when reconnecting the device, restarting the application is the user-side workaround I use for now.
STEPS TO REPRODUCE
1. Open dolphin.
2. Connect mtp device. (mtp mode selected)
3. Do any action.
4. Reconnect mtp device. (mtp mode selected)
The device is there, but contents of the mtp device are empty(no sdcard, no internal storage).
I have to restart dolphin to make it work.
The device and it is contents are available without having to restart dolphin.
Linux/KDE Plasma: 5.3.5
(available in About System)
KDE Plasma Version: 5.17.0
KDE Frameworks Version: 5.63.0
Qt Version: 5.13.1
I have done couple of tests, it just happens randomly on reconnection, there doesn't seem to be any form of a strong pattern.
possibly duplicated by Bug 440794 (which has a possibly useful log).
*** This bug has been marked as a duplicate of bug 440794 ***