The bug

In 1.12.2 /summon used to fail for falling_block if the specified block was not valid:

/summon falling_block ~ ~ ~ {Block:"invalid",Time:-2000}

In current versions this is not the case and there is not even an error message in the log.
For endermen the command did not fail unless you provided an invalid short (for example 32768s) but it used the outdated integer id system anyways.

How to reproduce

enderman

1. Try summoning an enderman holding an invalid block

   /summon minecraft:enderman ~ ~ ~ {carriedBlockState:{Name:"minecraft:invalid"}}
   

2. Try inspecting its carriedBlockState tag

   /data get entity @e[type=enderman,limit=1] carriedBlockState
   

   → It does not exist

falling_block

1. Try summoning a falling_block entity for an invalid block

   /summon minecraft:falling_block ~ ~ ~ {BlockState:{Name:"invalid"},Time:-2000}
   

2. Inspect its BlockState tag

   /data get entity @e[type=falling_block,limit=1] BlockState
   

   → It defaulted to sand