when you buy player, his game shape drops, usually between -1 to max -3... higher the game shape is, the bigger is drop...
DMI is strongly related to GS, so DMI also drops...
this feature was invented many seasons ago, trying to prevent managers to buy players only for 1-2 games (final rounds of national tournament games, playoff/playout)
now you have to plan several weeks before important games if you want to improve your squad...