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.
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.
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.