With my custom texture, all banners have odd-coloured pixels everywhere and are all white... Please note this is NOT a dupe of MC-66017, as all banners look correct with no resource pack enabled, so my Java and graphics driver versions are ok. Attached the banner texture file and a screenshot.
Kumasasa, I think you'll find that actually this is not a duplicate. The creator of MC-63485 managed to fix it by re-sizing all the banner textures to the same resolution, after which everything worked perfectly. For me, nothing will fix it.