SUMMARY
Long ago, I set up dual (identical) monitors connected to a "1c:00.0 VGA compatible controller [0300]: Advanced Micro Devices, Inc. [AMD/ATI] Cape Verde XT [Radeon HD 7770/8760 / R7 250X] [1002:683d]" video card.  At some point, I switched the two cables, which reversed the relative positions of the monitors.  I figured instead of crawling on the floor to switch the cables back, I'd just change the positions in the appropriate configuration section, and that info would be retained across sessions.  
 

STEPS TO REPRODUCE
1. log in to plasma with two monitors
2. using system settings, display section, switch the default to swap right/left monitors
3. exit plasma and restart (I do not use a DM, so it's not logging out, just starting a new session)

OBSERVED RESULT
 The monitors are displayed back at the default, not how I set them. (swapped right and left positions).  

EXPECTED RESULT
 The position of the two monitors would be remembered.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: kernel 5.6.2
KDE Plasma Version: 5.17.5
KDE Frameworks Version: 5.67.0
Qt Version: 5.14.1

ADDITIONAL INFORMATION
 I've tried both "restore previous session" and "restore manually saved session" (with an explicit save session) with the same results.

I know I'm assuming that this info should be saved across sessions, if I'm wrong, I'll just switch this to a wishlist to do so.

This might be related to bug 366739, but in my case, both monitors are always turned on, and the problem occurs on any restart of plasma, not requiring logging off or power off.

In case it matters, I use the right screen as primary, with the main panel on top.  The left screen is configured as desktop showing my ~/Desktop folder.  On restart, the monitors show the correct content, but the positions are wrong - the mouse flows of the right of the right monitor and on the left edge of the left monitor.