Sleep Sack vs Swaddle

Hi fellow Naniters! (is that a thing?)

My 4.5 month old little boy started sleeping in his crib recently. My Nanit Pro is working wonderfully but I am noticing that my son, who is still is a swaddle, flips himself over and ends up sleeping on his stomach part way through the night. In the morning I find him swaddled on his stomach with face to mattress