As far as I know, Withers are supossed to shoot blue skulls only when not having any target, and black when targeting some mob. I've had the rare situation in that the main head shot a blue skull, even if it's been shooting a chicken using only black skulls. I made sure, and none of the heads ever lost line of sight with the chicken.
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Possible Workaround and Extra testing
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All right, so...
I just spent the whole day testing this.
These are my findings:
The blue skull from main head you filmed is likely normal. The main head has always had a 1 in 1000 chance of firing a blue skull which is why most wither farms feature some form of fail safe mechanism or regenerating walls. (source: minecraft wiki and various youtube videos)
It Is true that the wither's side head spawn blue skulls randomly despite having one or multiple targets presented to them though. Which did not use to be the case.
This bug exists in version 1.14 - 1.14.4 (latest release at the time of writing)
and works as it always has in version 1.13.2 and below.
HOWEVER (and this is where it get interesting)
It works like normal in chunks that once were loaded in version 1.13.2
regardless of the version you are in.
Meaning; you can build a fully functioning wither farm in 1.14.3 as long as the chunks the contraption is located in were at one point loaded in version 1.13.2
I would not recommend loading your 1.14.X world in 1.13.2 though as it will delete any and all structures built in the spawn chunks and in the chunks the player loads himself.
Aside from that i also noticed a higher degree of instability after during and after conversion.
so be sure you do not have anything important there and make a back up just in case.
Extra info:
-I used a standard boat head lock and cobblestone wall tail lock for the wither cage
the targets i tried were witches and iron golems.
-I tried in all releases 1.13.2 - 1.14.4 but not the snapshots.
-I tried both multi and singleplayer. (no difference)
-I have not tested what happens when the wither leaves the 1.13.2 chunks and have not tested what happens if the target and the wither are in different chunks.
-I have tried in various world types, light levels and height levels, though as expected this had no influence.
If you have any further questions feel free to ask. I will be keeping an eye on this one as i need the wither locked and enslaved to finish my building project!
It takes some time to happen, but It certainly does. I've had it happening several times now. This bug makes it impossible to have a stable Wither rose farm, and any other Wither based farm where you need to keep blocks without blowing them up.