So looks like a breaking issue in Joomla 5.3.
I will look into it if it is still there when 5.3 is released.
Or you can post a bug on the Joomla issue tracker.
The issue was in the AMM configuration features > colors listed there did not have the # in the available colors. This site has been using AMM for over a decade and migrated from version to version. Perhaps I never notice before but I wonder if thew colors used to be specified like this before without #. Anyways fixed now.