That was a good workaround, it helped me figure it out
For others: what you need to do is to assign every player in the lineup to a fielding position before removing the DH. Even if it is nonsensical, like having an outfielder as C. Then you remove the DH by putting him to "-". That is the only position that won't get the pitcher auto-inserted.
Then put the position player you want in the lineup to P first, before putting the rest of the of the batters into their proper spot.
Thanks all!