It sounds like Braun Strowman is having issues with his knees.
On a related note, Strowman may also be dealing with backstage heat these days.
Multiple sources have reported that Strowman is negligent at times when it comes to showing up to work on time, according to PWInsider. Strowman reportedly also has a reputation of being one who leaves a little early and it's said that sometimes he makes mistakes in terms of etiquette backstage in the locker room. Another source noted to PWInsider that Braun is "his own worst enemy and his worst advocate."
Strowman has been close to the next step of a major push, with officials high on him, but then something happens and he makes some kind of misstep backstage, and that brings officials back down on him as they take a step back from his push.
There's no word yet on if these backstage issues are part of why Brock Lesnar won the WWE Universal Title at Crown Jewel but these are recent problems.