SharePoint search returning different results to Microsoft Search

I have a number of Communication sites that have an indexed site property called isStore that is set to true

I had a SPFx web part that was using the PnP library to search using isStore:true and this was working fine, until a couple of weeks ago when a few sites stopped appearing in the web part.

Initially I thought this was a development issue, but after creating an classic enterprise search center site I was able to reproduce the same problem.

Classic search

Modern Search

I have tried re-indexing the site and the document libraries, but this had no impact.

Does anyone have any ideas on what the problem is here?