For me, the only thing that worked was rebooting the phone .
I could launch the main task of the application, as well as debug the keyboard extension in another application, however it will not appear in the list in Settings -> General -> Keyboard -> Keyboards -> Add New Keyboard...
As soon as the phone was restarted, the keyboard extension appeared on the list immediately.
My assumption is that adding and removing a keyboard over and over can confuse iOS and cause the keyboard to already be added (hence why it doesn't appear on the list).
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