My daughter does this & she’s on cow & gate but I just help her abit to get it out by massaging her stomach & pushing down to get it out (when she’s constipated) otherwise she’s been fine & will do it herself. If she’s straining then maybe the formula isn’t helping her but the doctor recommends staying in a formula for a few weeks so the body can get used to it so don’t change it too soon
I believe you have to give formula a couple of weeks for baby to settle down when changing.
Infacol can cause more constipation, honestly I'd recommend talking to your GP or HV as she might need lactulose to help, my little one struggles but she's got cmpa and has just changed milk again so praying it helps her poop