There can be multiple causes for this, one of which is the backrest position. I know the Britax's seatbacks are a bit too slanted (not very upright), but the baby, perhaps, doesn't want to (or cannot yet properly) sit? If tired or not strong enough, it could just "let go" and slide down. What can help here is to have the baby in the lie-flat or at least quite reclined position, even if maybe the child wants to see - but it is healthier if it cannot do so by itself (otherwise, its spine and neck are overly strained, causing problems later in life).
The next thing that could be wrong here (or could at least help) is to have the harness belts properly, which means just above (or very slightly under) the shoulder height in terms of height setting (the holes where the straps are attached), and also just tight enough, not loose. It means, like with a car seat, tightened so that you should be able to get two-three fingers in between the child's body and the strap, but there should not be any excessive loosened,s then the strap lets the child slide down. Also, the waist straps and crotch strap, if possible, should be adjusted so they are just right around the child, not too tight and not too loose.
Lastly, the baby could be a bit too small for the seat (similarly the backrest position and the straps adjustment), just lost in the seat and having too much space around plus the shoulder belts too high. If that is the case, waiting a few months to use that pushchair OR getting an insert that would restrict the baby's space could help. A nice example oh which insert or cocoon to look for would be the Babymoov CosyMorpho (or try to search for "stroller baby cushion" on Amazon).
Some kids slide down purposely, just because they don't like the stroller or to be in (any) stroller, wanting for example to be carried in a carrier, in the arms, etc. This all depends on the child's nature, age, etc., about which I know nothing. I hope I listed as much as to help you at least a bit. As with many things with babies, a lot is just about trying (and failing) and trying again.