It won't cause dysplasia. If bub is already that way inclined, it could be that certain carriers make it worse.

I haven't seen the manduca up close but it looks like the main pressure would be the seat and not the legs - carriers like the baby bjorn force the legs downwards unnaturally. Carriers like the ergo, caboo etc put bub's legs in a more natural position and it looks like the manduca is very similar to the ergo. DD1 had severe dysplasia, DD2 very mild. I wouldn't have hesitated using an ergo and I did use a caboo.

I'm sure someone with hands on experience will confirm, but I wouldn't be rushing back for a refund just yet.