Blaze farms are broken by a change in blaze behavior. When the player damages a blaze all blaze aggro the player even if no blaze has line of sight to the player.
See attached example world.
MC-154100
All Blaze agro when one is hit even if none can see player (1.14 behavior change from previous)
Resolved
Duplicate
1
(Unassigned)
Unconfirmed
(Unassigned)
Minecraft 1.14.2, Minecraft 1.14.3 Pre-Release 2
Retrieved
Can confirm; all blazes in the vicinity aggro if you hit one, even if the one you hit doesn't have a direct line of sight at the player.
After experimenting with it, it seems it works much like pigmen aggro; as in, this bug won't occur if you kill the targeted blaze in one hit.
Hope this gets fixed soon.