I don't think there is anything to worry about there. The issue arose from the Knicks game, when Budenholzer benched Middleton for the entire 4th quarter and overtime (poor defense). He then missed the following game for "personal reasons." Middleton may have had something going on off the court that was affecting his focus on the court? He has always been a team-oriented, soft-spoken guy. So not too worried about that.
Bucks did the same thing the other night, playing a terrible Cavs team, when Giannis sat out with "neck soreness" and Connaughton was also given the night off.