It might have always been that way, you remembered it wrong and nothing ever happened in game to change your mind. I sure do it enough.
CoH 2 isn't amazing at conveying this sort of information to the player.
I noticed that the affected vehicle doesn't stop immediately so I might have just saw that and thought it wasn't disabled.
I don't usually get a M20 so I don't have a lot of recent experience with it. It is a munitions and manpower intensive unit on a faction that always struggles for both of those. The M20 isn't terrible by itself but doesn't really add anything to USF, considering that USF already has a lot of fragile, micro-intensive units. If you don't upgrade to armored skirts then it often dies when fausted if your opponent can follow up at all. The skirts should come with it and the cost should be in the 220-240 range. It loses badly to pumas and 222's so its window as a shock unit is already pretty small, especially if you upgrade skirts. A sight upgrade would make it much better, as would just 20 more HP so that it doesn't get wiped by double cloaked raketen's.