SUMMARY
As far as I remember position and size of widows started to be remember by default since some time. Basis on several sessions login/logoff I observed that Dolphin is not properly restored. I mean its size and position is not restored as expected.

STEPS TO REPRODUCE
1. Login with fresh user to Plasma
2. set restore session
3. Start Kate, Konsole, Firefox and Dolphin
4. log off and log in
   In result all should be restored properly
5. move dolphin to center of desktop and resize its windows significantly
6. log off and log in

OBSERVED RESULT
window is every time placed in left top corner (or in its original place after first start), and has small size (or its original size when was first time started) 

EXPECTED RESULT
position and size should be the same like set before logoff

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: YES
(available in About System)
KDE Plasma Version: 5.22.80
KDE Frameworks Version: 5.86
Qt Version: 5.15.3

ADDITIONAL INFORMATION
Dolphin 21.70.11

Maybe this is important.
Currently I have only one display, but sometime ago I had two and current one was lef. 
I found bug 387736 where is reported similar issue, and is related with displays.