this is from this past week's radio (on the mono buff song):
http://www.youtube.com/watch?v=-Pwp7yGllsEif you have a full colored grid and hit another colored block it will reset your grid with 2 blocks already on (hence not giving you a match21). this is on ironmode, but i tested on normal and same thing happened (2 block on grid reset after a full colored grid).
Doesn't seem to matter if you have a full grid and hit a grey - since the entire match then collects. Also if you just leave the match till it collects on its own it seems fine and gives you normal m21 bonus.