What I've noticed is that he seems to disappear when he has to play centre/CAM. Now this could be Harold space invading when he drops deeper but then I would expect him to go beyond Harold like a Dele or JLingz would in that position.
When he can play wider either as a side CM/AM/W he's much more involved in things. And is more of a threat in the half spaces.
That's exact what it is. He refuses to play like a striker for large parts of games and that impacts everybody else.
Half space Mount is elite. One of the very best at receiving the ball on the half-turn. His assists and goals should go up another level at club level with Lukaku as the focal point.