(Tested with vim 8.0.42.)
vimdiff these three files (there are no whitespace differences):
file-1:
111 aaa
111 bbb
file-2:
111 aaa
222 bbb
file-3:
222 aaa
111 bbb
The highlighting is wrong -- vim doesn't show the difference in the second line:

If you swap the lines in file-3, again the second (screen) line doesn't have its difference highlighted:

Note that (after restoring the original version of file-3), if you swap the lines in either of the other two files, vimdiff shows correct results. With file-1's lines swapped:

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I can reproduce this.
Closed #1186 as not planned.
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this seems resolved in the meantime.
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