It is more than likely missing something in the head section, open the main page and add a new text field and a summernote. See is anything new gets added to the head, then just delete what you added and save and test.
Some components seem to miss adding their parts to the document head and normally adding the component to the same file that holds the head data seems to fix it.
Worth a try.
After 2 days of work your suggestion to add the summernote directly to the layout page and then removing the textarea from the code resolved the issue. Having to do that isn’t great but I will accept it as a quirk and move on.