Let's call this a "bug", or actually a "documented feature".
According to the documentation, when you drop the partitioning index, the ending value for the last partition changes to "MAXVALUE", which is effectively an x'FFFFFF'.
Actually, this fixes a mismatch in earlier versions of DB2, since the ending value of the last partition was not enforcing; now it is, so "MAXVALUE" is actually the correct representation of the old mismatching behaviour.
You can always restore the original end value with
ALTER TABLE TST.TEST_TABLE ALTER PARTITION 2 ENDING AT ('ZZZ')