If this is actually an issue with the “Save Session” feature, maybe it’s worth looking into the ~/.config/ksmserverrc file. Mine works, so I’ll put it here as an example:
(Mostly) working saved-session from mine:
[$Version]
update_info=ksmserver_update_loginMode_value.upd:ksmserver_update_loginMode_value_default_enum
[General]
confirmLogout=false
loginMode=restoreSavedSession
offerShutdown=false
screenCount=1
shutdownType=0
[LegacySession: saved at previous logout]
count=0
[LegacySession: saved by user]
clientMachine1=localhost
command1=gimp-2.10
count=1
[Session: saved at previous logout]
clientId1=1010410efacb000168037641100000012700048
count=1
program1=/usr/lib/virtualbox/VirtualBox
restartCommand1=/usr/lib/virtualbox/VirtualBox,-session,1010410efacb000168037641100000012700048_1694301730_329206
restartStyleHint1=0
userId1=brian
[Session: saved by user]
clientId1=1010410efacb000172787082400000009200002
clientId2=1010410efacb000171608160200000009360003
count=2
program1=pulseaudio
program2=/bin/smplayer
restartCommand1=
restartCommand2=/bin/smplayer,-session,1010410efacb000171608160200000009360003_1728577777_652072
restartStyleHint1=0
restartStyleHint2=0
userId1=brian
userId2=brian
Something for me to dig through later and see if I can get my head round it!
Something to try, to help prove it, is back that file up, save a session, then temporarily rename that file & restore the backup you just made.
You of course won’t get your newly-saved session back next boot or logout, but you can compare the files between saved and non saved state. (Maybe post these here?).