SUMMARY
Apparently this behavior occurs randomly. Sometimes "CPU %" column of system activity (started with ctrl+esc) says "disk-sleep" immediately after Dolphin to complete the transfer of several files to another hard disk. Today I transfered 13 compressed (zip, tar.gz, etc) files (4.8 GiB in total) to my Home from another hard disk and this behavior happened again. Files transfer was completed, plasmashell process entered in "disk-sleep" state, Plasma panel disappeared, a few seconds later plasmashell process went out of "disk-sleep" state and then plasma panel reappeared. "coredumpctl" command indicated no crash related to plasmashell, drkonqi was not triggered. I repeated the same steps with the same files but the behavior did not occur anymore.

STEPS TO REPRODUCE
1. uses Dolphin to copy several files to another hard disk
2. 
3. 

OBSERVED RESULT
explained above

EXPECTED RESULT
plasmashell process never enters in "disk-sleep" state and plasma panel never disappears
after file transfers.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 5.17.1
KDE Frameworks Version: 5.63.0
Qt Version: 5.13.1
Kernel Version: 5.3.7-arch1-1-ARCH