Where it shows up
Banding needs a large area changing slowly. That is why it appears in skies, gradient backdrops, out-of-focus backgrounds and the falloff around a light source, and almost never in detailed texture, where the noise floor hides the steps.
Two separate causes
Bit depth. 8-bit gives 256 levels per channel. A sky occupying a fifth of that range gets about 50 steps to cover a gradient a few hundred pixels tall, and the boundaries become visible. 10-bit gives four times as many levels for the same range.
Compression. A codec that allocates too few bits to a flat area will quantize it flat, creating bands that were not in the source. This is why the same clip can band on one platform and not another.
Spotting it before you convert
Banding is easiest to confirm on the file rather than by eye, because a display's own dithering can hide it. Check the bit depth first:
ffprobe -v error -select_streams v:0 \
-show_entries stream=pix_fmt -of default=nw=1 input.mp4
yuv420p is 8-bit, and 8-bit is where source banding lives. yuv420p10le is 10-bit, where it usually does not.
The one that matters for conversion
Banding already present in an 8-bit source is baked in: the levels between the bands were never recorded, so nothing downstream can recover them. Converting to HDR10 will not remove it and may make it easier to see, because the highlight range it sits in gets stretched.
Banding introduced by a conversion is a different matter and is avoidable. This is why the output is 10-bit rather than 8-bit: remapping into a wider range at 8-bit would create banding that was not in the original.
There is a real trap in verifying this. VP9 and similar codecs do not treat a quality setting the same across bit depths, so an HDR rendition encoded at the same nominal quality as its SDR counterpart can come out with substantially fewer bits and lose exactly the highlight detail the conversion added. If you are comparing two renditions, check that the HDR file is the larger one.
Questions people ask
Can banding be removed after the fact?
Not honestly. The levels between the bands were never recorded, so anything that smooths them is inventing data, and debanding filters work by adding noise to hide the steps rather than restoring detail. They can help perceptually and they do not recover information.
Does converting to HDR make banding worse?
It can make existing banding easier to see, because the range those bands sit in gets stretched. It does not add new banding, which is exactly why the output is 10-bit: remapping at 8-bit would.