Multi Tenant Organization - one shared user not showing up in Global Address List

To add to my previous post, I have discovered two things.

1) There is more than one user that this has happened with (I had only previously spotted one)
2) I seem to have found something that correlates with the external member visibility in the global address list

It is when I look at the account in entra, if I go to the overview of a user that DOESN'T show in the GAL I see this

Whereas if I go to the overview of a user that DOES show, I see this

So, the difference is the Sign-Ins box, which indicates that there is some setting that is different. Hopefully someone can tell me what the difference between those two accounts is? (They are both shared from another tenant in an MTO, there were some issues getting the connection working properly so they may have been shared with different settings)