A lot of people kinda missing the point which is, even if KDE follows convention by copying memory to RAM and syncing to disk slowly (which is always gonna happen, USBs are slow) Dolphin should properly identify this and not just say “we did it! the transfer is complete!”
That being said, Nautilus (GNOME) does the exact same thing and it’s always so annoying I have to search the web every time I want to remember how to track the progress of the sync command.