Securing Azure Function backend api with AzureCloud service tag - change in behaviour

I have six SWA instances deployed to West Europe (Standard SKU). These all follow the same architecture:

SWA (React FE) -> /api route -> Azure Function (BYO, nodejs - deployed to UK South)

The Azure Function inbound traffic is restricted with "AzureCloud" as the source. This is working fine for the six instances - I cannot reach the function app directly unless I go through the /api/.

A couple of days ago I deployed a seventh instance - exact same architecture, same region etc.

The Azure Function returns a 403 if the "AzureCloud" service tag is present in the inbound traffic restrictions. If I remove the rule, the function app can be reached from the SWA as per the other six.

Debugging the issue, what I can see is that a header is returned in the 403 response for the sixth instance:

"x-ms-forbidden-ip" - (starting [fd00:0:1200)

Testing this against a working instance, I get a different result:

"x-ms-forbidden-ip" - 13.69.116.3

As you can see the above ipv4 address is Azure owned, originating from West Europe - which is expected:

Aside from the firewall seeing an ipv6 address over an ipv4 address. Why doesn't the "AzureCloud" service tag work?

My guess is that the ipv6 address isn't Azure owned or this is missing from the published IP ranges.