After doing a lot of banner crafting without closing the GUI sometimes the little icons on the buttons in the middle start getting overwritten by the previews (left to right, top to bottom). After it starts, each time you click a different banner element button or swap out the dye (I didn't test swapping out the input banner), the next icon is replaced with the current preview.

It first happened to me by accident while I was playing on a Realms server.

I tried many times in a single player world to recreate it and take a screenshot. (attached)
I'm not sure what determines whether it happens or not, since I tried making the same set of banners as my screenshot for a second time but it didn't happen again. Also in some attempts I made many more banners than this and it did not happen.

When I did manage to get it to happen, I had kept the buttons active for the entire crafting time (the buttons appear when you have a banner and a dye in, but disappear otherwise), then it started happening when I removed the dye and put it back in. (but I don't know whether these conditions are required)