This is without a doubt due to potted plants not having an assigned item due to 1.13's refactoring, but this behaviour seems very wrong. Although endermen drop empty flower pots when killed, they drop neither the pot nor the plant when killed while holding a potted plant.

Steps to reproduce

1. /summon minecraft:enderman ~ ~ ~ {carriedBlockState:{Name:potted_bamboo}}

2. Kill it
   → no pots or bamboo are dropped

3. /summon minecraft:enderman ~ ~ ~ {carriedBlockState:{Name:flower_pot}}

4. Kill it
   -> The pot is dropped

Expected Result

The expected behavior would be that endermen holding potted plants would drop the plant and the pot upon being killed.