@SophieAmnesty @jaymcp will correct me if I am wrong but I think thats due to a feature on .org that you cant be both, research and news and impact etc? When they migrate in they will have the same taxonomies etc

Ahh ok, so any of the sites still using multiple lists of terms within the Content Type taxonomy are going to need to change to multiple separate taxonomies.

I've had a look through the sites this morning, I think the sites would be the Chinese and Persian IS sites (which would be good prep for site migration into to .org anyway) and then Ghana, Nigeria, Philippines and Zimbabwe. https://oneamnesty.sharepoint.com/sites/AISFNCWebDevelopment/referencedocuments/Taxonomy%20Audit%20-%20IS%20Hosted%20Sites.xlsx?d=w710e9862e6d5469c8aedb102ec1d28a7&csf=1&web=1&e=eFcGQj

Sorry for the late reply on this; not sure how I missed it.

It seems that, for some reason, the inability to tag a post with more than one category is a bug triggered by the branding plugin.
In the plugin, we remove block patterns that aren't ours (e.g. WooCommerce, Yoast, etc.) to reduce noise in the editor, and provide a better experience for the editors.
However it seems that, in unregistering those patterns, somehow WP can no longer load in the full list of terms assigned to the post in the editor.

This makes absolutely no sense to me whatsoever, but at least I have a starting point to figure out a solution.