SUMMARY
this is really two issue working together to create a curious annoying behavior.
When umount and remounting the same device, an empty tab will be shown if you did not close the tab of the unmounted device; I think the issue is that the temporary device folder name is the same, and thus dolphin get confused

STEPS TO REPRODUCE
1. Connect and open a removable device
2. Open it in dolphin (root folder of the device)
3. Unmount it (now dolphin will show an empty folder, still at the location of the device), eventually minimize dolphin
4. Remove the device, and plug it back in, select again to open in dolphin

OBSERVED RESULT
the tab with the empty folder is shown. If dolphin whs in the background or minimized, it will be correctly bring in focus, so it 100% seems like the device is empty!
To correctly visualize the content, you need to click again on the device on the side panel; try to F5 the folder will instead return a "folder does not exist" error


EXPECTED RESULT
Or a new tab is open, or the old tab is correctly refreshed to the new folder; but I would expect as real fix that the unmounted device folder would be simply closed, as they are invalid anyway


SOFTWARE/OS VERSIONS
Operating System: Arch Linux
KDE Plasma Version: 5.19.5
KDE Frameworks Version: 5.75.0
Qt Version: 5.15.1
Kernel Version: 5.9.0-1-mainline
OS Type: 64-bit
Processors: 12 × AMD Ryzen 5 3600 6-Core Processor
Memory: 15.6 GiB of RAM
Graphics Processor: AMD Radeon RX 5700 XT