How do I restore missing Favourites on one device?

As described, you may have a corrupted file that resides within your Safari cached data - this file containing the website icons that appear on tabs and the Safari Start Page. This file may simply need to be deleted from your cached data; this file will be automatically recreated by your iPad. Deletion of this file does not delete the actual saved Favourites from your iPad:

Settings > Apps > Safari > Advanced > Website Data - use the Search dialogue to find data for "apple"

Identify the cdn-apple.com entry; with your finger, swipe the entry to the left to expose the associated red Delete button. Delete this entry (and just the entry) from the list.

Having deleted the entry, you can leave the settings page - and restart Safari.

I guess you updated to 17.7.7 recently?

Same/similar problems with iPadOS 17.7.7 (iPad 10.5; German), see other posts here in the iPad Community. Very annoying!

Like ones you have had and used for years?

Favorites what?

Just remake your favorites. That simple.