Yep, that’s the solution - well, part of. I spoke with Autodesk as I hadn’t twigged what exactly was the issue…
If you Perspective Match with a free camera (which is the only camera you can Perspective Match with), then when you do a “Create Physical Camera from View” you have to then uncheck ‘targeted’ for things to stay put.
If you “Create a Physical Camera from View” from a Targeted Camera view, then leaving your newly created Physical Camera as Targeted will be fine.
Basically, it’s all got to match.