Using Twin Universe on a diving mutant gives the resulting copy similar stats to the original's BASE stats, not the doubled attack. This happens every time with mutants now, though diving pegasi still copy correctly. In this case, a 12/7 Azure Dragon was copied while diving for 24 atk resulting in a 14/8 copy during a PvP2 match.
I recall diving mutants copying correctly before, so the bug is probably in either the 1.30 or 1.31 patch. On a side note, it took a while to get the perfect screenshot of this bug in action
.