I had this issue. I tried to get fancy with my template and combined all the css into one file for fast loading. I didn’t leave the style.css in the main directory which is used by wordpress for information about the theme.
I noticed that the block in the admin which lets you select which template to use for the page was missing. This lead me to the issue with the theme.
Note that my AIOSP meta information did save for pages using the regular page template. It was a really weird issue to trace so I thought I would share here in case others stumble on this page.
