Supermarket transaction not listed in banking app

Armorica

The devil is in the detail.
The floor limit is 0.00 - vast majority of contactless transactions are online and authorised by the bank. The key exception is for travel - where the connection may not be available and these are handled as offline in some circumstances.
The original transaction mentioned was £55 so doesn't sound like it was contactless though?
I believe stand-in processing is still around though - and this is when Visa/Mastercard can make decisions on behalf of the card issuer within set parameters. This helps smooth over cases when the card issuer doesn't respond quickly enough/is offline. It helps everyone cope with the traditional christmas peak volumes.  You may have just bumped into that.