It might have something to do with polling rate. If your mouse has custom software settings, lower it to 125hz (You will probably also have to adjust dpi and acceleration to reduce speed). This solved my mouse problems during matches. (It used to slow down).
I think starting in 10.13 apple has tightened security regarding clicks and cursor location and the result is that default values can override custom values for mice under certain circumstances.
I urge the devs to look very closely to what happens after a match ends and the stat screen comes up (Mouse speed ALWAYS is greatly reduced right there all of a sudden). And maybe come up with a theory why this happens and sort it out.
This also seems to be the case in some other Blizzard games.