It is known that the "Upgrade to New Db2 Releases, Modes, and Function Levels" in the Database Management for Db2 for z/OS product documentation
should be reviewed and completed when upgrading a Db2 releases, Modes, and Function Level.
The Post Install tasks were not performed after a Db2 fallback from v12 to v11. Db2 v11 after the fallback was able to run without problem.
(The SETUPxx member in hlq.CDBAPARM was changed after the fallback.)
Are the Post Install tasks required to be performed after the fallback to Db2 v11 from Db2 v12?
If you work with a SETUPxx for Db2 11, the Post-Install Tasks can be omitted after fallback.
Plans do not need to be updated as you will work with same SQL features for Db2 12 FL100 and Db2 11 NFM.
DB2 objects in the PTDB database are designed to work correctly after any fallback so it is not necessary to run any Post Install Compare.