I don't know, It's only the base colour that changes, the rest of the patterns applied to the shield don't change.
Tried it with my resource packs and no resource packs at all. The resource packs are irrelevant to this bug.
There you go, freshly baked crash report.
I don't think this is a duplicate, although similar, the item isn't required to have been named previously in order to cause a crash.
This is my single player worl, but like I said, I suddenly remember that I had been there before and tried to cancel the bug submission but didn't know how.
Reloading chunks doesn't work. And I don't think it's related to MC-91135, Because the lighting doesn't get delayed. It never passes over until a block update on the chunk border. Placing the light source, it's normal, it's when i leave and come back to the chunks that it happens.
As this doesn't seem to be the same, although similar, I don't think the issue is resolved.
This is intentional behaviour. The don't need a spawner to spawn in a fortress.