Why is c4 bad when playing the London against a King's Indian? Recently I (rated ~1900 "classical" on Lichess) played the following opening:
1.d4 Nf6 2.Bf4 g6 3.e3 Bg7 4.Nf3 O-O 5.Nbd2 d6 6.c4

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Upon playing c4, the stockfish analysis goes from a score of 0 to a whopping -1.1, as if I've already blundered a pawn. I've noticed this in a lot of my games - when black avoids developing his queenside and putting pawns in the center, I'm tempted to play c4, planning Nc3, castling kingside, taking up space on the queenside and if black is passive, perhaps pushing for a central pawn break with c5. But in these situations stockfish usually strongly dislikes c4. 

Why? Stockfish's response is usually suggesting black play c5 pretty quickly, but this allows d5 giving me a lot of kingside space instead, and I can imagine a nice eventual pawn break with e5. I'm perhaps not at a level where I should be focusing on the opening so much, but I feel like I'm missing a big strategic idea here.
It may be to the fact that you are making his bad Bishop into a Monster.
After 6...c5, 7. d5 Nh5!  and the dark square bishop is free to roam.

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