I think this is caused by a) the fact that due to the inserted references the reference indexes of all other refs changed, and b) the timeout that prevents proper diffs inside paragraphs.

E.g. in https://de.wikipedia.org/w/index.php?title=Internationale_Mathematik-Olympiade&diff=149908931&oldid=147676407&diffmode=visual&visualdiff=1 a reference was inserted, too, and all the following references show up as changed (and I still think this isn't the best solution, but I can remember some discussions about that, though I neither know where they occurred or what the outcome was), but in this case no timeout happens and thus the diff is somewhat reasonable.

Actually, it's just the same as T196380: Visual diff shows unchanged text containing a reference as removed/inserted (absent from source diff) , though the reason why the timeout occurs may be different.