iphone 15Pro backed up ios 18.1, restored to 17.7 normally, then backup not found

As the title, I backup the phone like normal while it was 18.1, downgraded to 17.7 using itunes restore function, as the normal process works, and now itunes says "this phone has never been backed up to this computer before", it was treated like a new phone that had no account tied to it. edit: searching the %appdata% folder shows the 18.1 backup file there, but none others. Does itunes only keep one backup ?

I'm sure nothing was skipped in the process. And surely if ios18 had some restrictions on downgrading it would be made public by now?

Even this post is showing "Admin's iPhone" in the button to select which device I'm posting about. This seems all very odd.