Using commands to summon goats as screaming goats seems to only produce screaming goats randomly as if summoning them without the born_screamer entity command. 

Repro:

Use either /summon minecraft:goat ~ ~ ~ minecraft:born_screamer  /event entity @e[type=goat] minecraft:born_screamer to summon a goat. Kill the goat. 

Expected:

All goats summoned or converted to screaming goats with these commands should scream

Actual:

Most goats make the normal sounds. Some goats scream, and it's unclear if this is just the normal random chance for goats to be screaming goats.