Header problem in Apple Pages 14.2 running on Sequoia

This is related in some way to the Styles function. When I changed the Style of the page number, the header's three fields appeared and I was able to edit the left-hand field. I could then change the Style of the page number back to 'Header and Footer'. So I've found a workaround but this is a bug of some sort.

Weirder and weirder. I can only edit the left-hand field in the header if I first change the Style of the page number in the right-hand field.