Using rats-6.0.4-1.16.1.jar

Spawn a rat
Tame it with cheese
Give it an angel upgrade and a stack of diamonds
Kill the rat

The rat will respawn with the items, but a duplicate set of items will drop as well.

After examining the code, it seems like the 1.16.1 LivingEntity class spawnDrops method calls func_230282_cS_(), a duplicate of canDropLoot(), rather than the actual canDropLoot(). Thus, the override in EntityRat doesn't work as intended. Submitting a pull request shortly.