Pin to top, disabling thumbnails

I have a few document libraries on different sites with documents "pinned to top". This is a really nice feature I believe and it has worked flawlessly up until now. For some reason documents pinned to top are no longer displayed with a thumbnail image but instead with the app icon only. Moreover if I set the library to show tiles, then all docs are displayed with thumbnails except those pinned to top.

This behaviour is for all my libraries/sites and if I create a new site it get the same. It does not matter which browser I use and is even the same on my phone.

I have no idea what caused this - and MS support came up short after an hours remote session.

This behaviour started two days ago and might be connected to another issue I have with Highlighted Content Webpart https://techcommunity.microsoft.com/t5/SharePoint/Highly-Confused-by-Highlighted-Content/td-p/329669

But I fail to see how changing the "Title" managed property on two sites can have this effect on thumbnails - tenant wide.

Any ideas on how to troubleshoot this are highly appreciated.