How to reproduce
Give yourself a stack of empty banners
Give yourself some banner pattern
Give yourself a stack of some dye
Insert all 3 into a loom
Left-click the output slot and observe the pattern selection
See attached screenshots.
MC-137627
Loom pattern selection turns blank when using banner patterns
Resolved
Fixed
6
Confirmed
loom
Minecraft 18w43b, Minecraft 18w48a, Minecraft 18w48b, Minecraft 18w49a, Minecraft 18w50a, ..., Minecraft 19w04b, Minecraft 19w05a, Minecraft 19w06a, Minecraft 19w07a, Minecraft 19w08b
Minecraft 19w09a
Retrieved
Only happens if the input slots have more banners and dyes than one item.
So if there is just one dye or just one banner, or both one banner and dye, crafting a banner pattern with those will not turn the pattern selection blank.