It seems a bit of the issue might stem from how long turns represent. Unless I'm mistaken a turn is four days. Having mud hold up an advance for that long would seem to be a bit much during the summer months. Brief, heavy, and violent cloudbursts were (and are) a fact of "steppe" life. So too, however, is wind and heat. Several German generals - including Halder - talked about the summer mud holding up advances, however, these hold ups tended to be brief - particularly in comparison to the "mud season".
It's not a one to one correlation, of course, but living in the "American Steppe" I've seen violent thunderstorms completely flood out areas in the space of an hour. In the storm's wake, however, comes the heat and wind that will completely dry it out within a few hours or so more.
I completely "get" the reasoning for mud during the summer rains. It's a historical fact. I would contend, however, that there's a difference between the mud of a few hour thunderstorm as a front moves through and the mud of the Rasputitsa - particularly over the course of four days.
I don't know the feasibility, but my suggestion would be to add a "summer mud" terrain type which could run up through, say, August. "Summer mud" would have the penalties of "mud", but halved, quartered, or something - obviously testing would work that out. This "summer mud" would thus represent the day or two a unit was slowed down by a cloudburst and resulting quagmire, while also simulating the day or two that was "normal" conditions as the heat and wind dried it out.