Just looked into it - this can be fixed if gamerule universalAnger is set to true
Confirmed to still be a problem in 1.16.2.The AngerTime tag is completely ignored when custom set for any neutral mob. Doesn't matter if it's spawned/summoned, has the data modified, or has specific UUID set to be angry at. Reverts back to zero.
Just looked into it - this can be fixed if gamerule universalAnger is set to true