I am currently trying to setup an individual custom theme specifically for some products. I have another custom theme which will style/structure the rest of the site along with its own product page styling. However, this doesn't seem to work. It still uses the structure from the "master theme" and only takes the styles/assets from the selected alternate custom theme chosen inside the products admin in Schedule Design Update.

Here is the structure for the "master" theme which structures everything BUT the products that get told otherwise.

This theme is working as expected and, whilst testing I dropped in the XML file to this theme which structures the rest of the site, it creates the expected layout. Here is the structure for the "alternate" product theme, which the XML doesn't seem to get recognised at all, no matter which XML file inside Magento_Theme > layout.

Can anyone spot a problem in how both themes have been setup? The "alternate" theme is not a child of the master theme, however this has been tried too.

Thanks in advance!