I have been trying to test iOS online subsystem functionality for my game and thought that this blueprint node was just broken. This bug will probably catch many UE4 developers that are new to iOS because of how specific it is. If the developer is testing on an iOS device that has ever enabled Game Center even once then it looks like this bug won't appear. I'm only assuming that the expected behavior for the node should prompt the user to log into Game Center since that is how the node behaves after having enabled Game Center once AND THEN disabling it. Currently the work around to this seems to be that iOS developers will need to put up an error code and then a description prompting them to manually enable Game Center through their device's settings.
I also had an iPhone 6s that had never had Game Center enabled and was able to replicate this issue on that device. The same behavior occurred where I would only get back failure from [Show External Login UI] - After toggling Game Center on the issue was fixed and if the device was not logged in it would prompt the user to log in. So I've been able to test this issue on iPhone 6s and iPhone Xs but I will be unable to test anymore since I have no more available devices and I can't factory reset these devices.
You must use an iOS device (Tested on iPhone Xs) that has never had Game Center enabled on it before. (This may mean that the device will need to be factory reset/fresh)
1. Attempt to use the External Login UI Blueprint UI Node and test on an iOS device
Result: The blueprint node will just return with [Failed] without prompting the user to login.
Expected: After enabling Game Center on iOS device the blueprint node behaves as expected. However, when Game Center is then manually DISABLED by the user and this blueprint node is called then in game the user will be prompted to log into Game Center (Which is also expected). This means that on a device that has never had Game Center enabled there seems to be a bug where it will fail to prompt the user to log in when calling [Show External Login UI]
Head over to the existingAnswerHub thread and let us know what's up.