I have three stacks of illager banners that won't combine with each other, like this:

• 1x Illager Banner
• 2x Illager Banner
• 4x Illager Banner

Each stack shows the following tooltip:

Cyan Lozenge, Light Gray Base, Gray Pale, Light Gray Bordure, Black Fess, Light Gray Per Fess, minecraft:white_banner, NBT: 2 tag(s)

Even though they all appear to be identical, I cannot pick up one stack and combine it with the others.

My theory is that something in the NBT tags is different so they won't stack, but what could it be? I'm not 100% sure, but it seems that with every update to a newer snapshot, newly acquired illager banners won't stack with older ones?