There’s an option for 'starting in full-screen ’
That works with standard landscape videos, if you try with a portrait short you have to turn the phone twice to have it in fullscreen.
In my experience only Libretube works correctly with auto fullscreen portrait videos.
Yeah, unfortunately I had to move out yoo of libretube. Its a great product, but I found out thst I was missing more than half of my subscribed channels. Feeds come from the instance, and say you are subscribed to 20 channels, different instances will show you different sets of 5-10 channels out of 20 
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It works for portrait videos as well for me.
That should be the case only if you have auto rotation turned off and I need it on for other apps.
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