* any attempt to modify printer instance settings using "lpoptions -p laserjet5si/konzept -o InputSlot=Upper" results in deletion of all "Dest" lines previously present within lpoptions file and modification of the single line that was considered to be affectedNot quite my observation. I cannot set up more than one instance. Adding a second one witj lpoptions deletes the existing one. You show five lines in your example. Was ~/.cups/lpoptions edited by hand?
For clarification:The ~/.cups/lpoptions file cited above has been extracted from a backup taken before upgrade towards debian bullseye (minus potential but in case present unintentional modifications while copy-pasting from the backup). It is being rewirtten in the same way you reproduced above upon any attempt to issue an "lpoptions -p" command with the new CUPS installation.
Thanks for keeping tracks of this issue. Regards, Martin