Phone number format randomly changes

My iPhone 7 is set geographically to the UK with all the attendant formatting (dates, phone numbers etc).

Every now and then, randomly that I can work out, stored phone numbers in Contacts change format. They go from UK ( 0111 1111 111) to something quite different (01 11 11 11 11 11).

This then means that received calls and text messages don't match the phone numbers in contacts and so all incoming calls and texts are "unknown."

The only way I have found to solve this is to change the geographical setting to another country (I choose Belgium, why not?) and then change it back to the UK. This then fixes all the phone numbers.

If I don't do that, even when I edit the phone number it stays in the wrong format.

I don't have a Mac and the phone only syncs with iCloud (which seems to remain OK throughout).

How do I stop this happening? It is somewhat annoying!