Both the emacsformacosx.com versions and the homebrew cask version output the same message to stderr. These are not identical versions (executable size is different, unclear what is included or not included in each) but both claim to be 27.1.
I'm also convinced that this message is coming from the OS/GUI, but planarGetBytes is not documented anywhere so I wonder if it some sort of low-level constructed method call.
I've seen reports of other users seeing similar errors over the last several years, but the precise message contents don't always match. "count (N) is less than bytesPerRow (M)" where N & M are different.
I'll try a full build from source and see if it still occurs. If I could find a way to generate a stacktrace when this happens then I might be able to track it back to the particular emacs event. Currently I can't find any correlation with my usage.