Well that is strange. There is a difference between the two partnumbers but the older one should only be incompatible with the C9300LM versions.
This is the screenshot for the Cisco mode C9300 stacking cables.
This is also present in a Cisco community post.
So it might be that the -M variants also use a slightly different version of stackwise-320. I would hope a Cisco engineer would explain the difference between the two.