It looks like I solved the issue but I think I may have also found a bug.
I didn’t work any more on my project since posting this yesterday- This morning when I woke up the first thought I had was to enable Game Center for my iPhone Xs manually through my settings. After I did this the blueprint node this morning worked perfectly. I was even able to test it on a build that I pushed through UE4’s ‘All_iOS_On_Desktop-…’ compile function for iOS testing.
So one of 2 things were the fix- Either I just needed to give it a day (I really doubt it) OR it was the fact that I enabled Game Center manually.
But what’s really odd- Is that after I tested it and got [Show External Login UI] to work I went back and manually disabled Game Center. THEN THIS TIME when I went to go test the blueprint node Game Center appeared and prompted me to log in. So this means that Game Center needs to be enabled at least once on a device for this node to work.
I think this may mean there is a bug with this node being unable to prompt iOS devices that have never enabled Game Center to be able to login- It’ll just fail. Unfortunately this is a time consuming bug to confirm- I can’t confirm it myself because I don’t have any spare iOS devices I can factory reset to confirm that this is the issue.
The work around to this is to just prompt users to display an error code to your users and to prompt them to try enabling Game Center manually on their device. This isn’t the most elegant solution though.
I’m also going to be submitting a bug report. I’ll post back here if I have any updates about this issue.