Looking at this joomla.stackexchange.com/questions/31618/… I also think that J3 was helping me out by appending Itemids in a way that J4 no longer does?

Fixing all the internal links sound like the best solution, as you suggested – none of them worked when we upgraded from J3 either so I know where they all are.

Thank you for taking the time to write this down – it’s very helpful. The key thing that I did not understand was that if an Itemid is missing the home page ID will be used by default, although I suppose that should have been obvious.

The module I want to display on the home page is appearing on ALL pages that are linked without using an Itemid. I have other modules on different pages and they don't show up in the wrong place - it's just an issue with the home page.

In your article you say: “If you remove the ItemId from the URL of a certain page, then only those modules assigned to all pages or those assigned to all pages except those selected will display on that particular page.”. But this is not what I am seeing.

Thank you for the helpful pointer. I also found your article here itoctopus.com/… which explains a bit more.

Thank you so much! Yes, I can make the Joomla 3 site fail the same way if I change the routing to 'Modern'. Unfortunately removing the ID didn't help, but at least I know what I need to look at now. I have reverted the main site to J3 and now have two test sites, one at J3 and one at J4 so that I can keep experimenting.