I think it is mental, but shouldn't be taken in isolation. In other words, I think he's losing these big points because he has no confidence that he's playing well enough to play them on his terms, so he's not fully confident with his ground strokes (forehand in particular) which makes him hesitate to pull the trigger or be as aggressive (he's had plenty of DTL forehands perfectly lined up and he hit them more towards the middle rather than go for an outright winner). So yes, that is mental, but the mental block on these points is a result of him just not playing good enough. The return is poor, so he's not having many opportunities to break his opponent's serve, which in turn is making him tentative so he's not really going after the points. It feels very reminiscent of 2013. Also, much like that year, his movement seems off. A clear split step slower and that changes things drastically. The domino effect entails and all of a sudden, he's not confident in his ground strokes, return, etc... which changes his tactics, his approach on the big points, etc...
Where I would agree that the mental block is costing him, is that he's not being a good enough front-runner. He won the first set, as he did against Millman at the US Open, then just failed to ride any wave of momentum. Plus, when he realizes that his opponent isn't going anywhere, that's where he seems resigned to a close match and starts going in "I have to find a way to navigate through this match" mode, rather than "I just have to play my game" mode.