I have tried that, as well as ensuring all textures have the same bit depth, etc. The issue still persists, even going so far as to make all banners regardless of base colour to be white. This issue happens at any lower or higher resolution – 32x, 128x, 256x, etc. I understand mixed resolutions would especially be a problem, but I have ensured all 40 banner textures once edited are the same size. Unfortunately, no situation yields properly rendered banners.
I would like to emphasise that this isn't just higher-resolution textures; the issue persists with lower-resolution textures as well, which are also supported by all other entities.
I am unaware of any other entities that aren't capable of rendering textures of other resolutions. Banners seem to be a unique case. I imagine this may be due to the colour mask layering to create patterns ? I don't see why it cannot support it like any other entity, but I digress.
Oh ! I have just discovered a single texture that makes or breaks this whole issue: the banner base texture (which is not included in the banner entity folder and simply rests outside of it) must also be resized. How bothersome.