The real world is not a black and white place but the wrestling world certainly should be.
Reigns is not a tweener. Austin was a tweener, Rock was a tweener. A tweener is a face that isn't above acting heel but there are 2 key elements Reigns lacks to be an actual tweener.
1. The fans don't react like he is a face
2. He cuts heel promos intended to get the fans to boo. Austin or Rock did not do this as tweeners. The would insult other performers, sure, but they never said anything in a promo that would get the crowd to boo them.
At this point it seems a point of pride more than anything. Vince wants to validate Reigns' booking by proving he is the top face Vince thought he was all along. It's just weird to me. They are perfectly ok with Reigns trying to get heat but they will never officially turn him heel. It's just dumb and it does not work.
Take this feud with Strowman. How were we supposed to react? In kayfabe, both men are terrible people. Strowman gets cheered by smart fans because he is whipping Reigns. Reigns gets cheered by the kids because he is the top face but booed by everybody else also because he is the top face.
How am I supposed to care who wins? Neither guy's character is good in this. Take out all of the knowledge of the business and look at the feud through the lens of a mark. There are no stakes. Either way, a terrible person wins the feud. That's just terrible storytelling.