So assuming that I then lost the next game with the same party against the same enemy team, my MMR would fall by -22? That is just a net gain of +6 when I was clearly playing against people with substantially higher average MMR. So to chase my MMR up from 3100 to 3400, I would need 100 of such games played, assuming a 50% win rate? That seems like an exceptionally flawed system to me.
And based on my reading of the dev forums, it seems like you will always lose MMR for losing and gain when winning.
If you have a 50% winrate then the MMR has got you in roughly the right place for your skill. So such a gradual increase is fine as you are only gradually going to get better. If for some reason you should be a higher MMR than you are then you will have the winrate to prove it.