Hey, I'm sorry for not seeing this support thread earlier.
I am unable to find any issues relating to negative MMR being granted, however I investigated a similar report recently and it was found to simply be a display issue.
If it ever happens again, please provide us with the date / time of the match, as well as a short recording of the match summary if possible. We can then double check your stats to see if it counted for a Win / Loss and if there was a change of MMR.
As I say this is likely a display issue and I'm sure you did in fact receive positive MMR