Thought at first it was exploit but after watching the replay again it seems to be like a weird bug.
In the replay it concerns Artifax's front 2 mexes. And happens around the 13 minute mark.
Artifax upgrades his mex to t2 and surrounds it with storage before starting t3 upgrade and pausing. To Artifax it's displayed a T2 mex on a paused upgrade. To everyone else (including his own team) it's displayed as a T3 mex on 22 HP. On attacking it looks likes its losing no health and is stuck at 22HP but to Artifax its a T2 mex losing normal HP.
Same as the Mex behind it. Looks like a T3 mex on 27HP.
uploaded replay