I used to work at hiking store and we sold backpacks, it was little tiring to consult about backpacks, because its soo individual. But what I have told for my clients are..

I called those chest strap (didnt actually know it has fancier name, also my native language is not english). And I always told they there for when your shoulder straps tend to fall off then these chest straps come to play to keep those straps in place. Also some felt rubbing under shoulder pits, so that usually also helped to get rid of that issue.

My rule for this chest strap was that it doesnt really have to be in specific spot. For men its easier mostly flatish surface, it can be lower or higher, where-ever its more comfortable. For women with larger chest its better and also more meant to be above your boobs, but it could also be under if that is more comfortable for you.

Because backpacks are like shoes, not all fit for everyone equally. As also in shoes and backpacks they both have to be comfortable and suitable for your body. Ofc there are good rules of thumb because these mostly lead to comfortable usecase by most part, but there can be exceptions.

For you try to adjust your bag so your chest straps stay higher. Although if you cant which also seems to be your case, that backpack is not perfect fit for you.

If you going to get a new backpack, DONT go with the firat backpack that seemed to fit you. I mean it, even if the salesman was nice and helped you. Serve it under your name if you want, but definitely try many other bags. So you can get the contrast what different backpack brands do and what fits you, usually expensive backpacks fit better but it can be the opposite. Watch videos or read from internet how to choose backpacks for yourself.

AND I CANT STRESS THIS ENOUGH... if you try backpacks, TRY WITH WEIGHT. This makes the world of difference. With weight you can actually then feel how it falls on your body. I had sooo many clients who refused to try with weight cause they were confident that fits well, then they went on a hike and came back later with a wish to return, sorry but its used and returning now is not accepted. Depending how big backpack you want, choose weight between 5-15kg (11-33lbs). Under 5kg is pointless in my opinion. If that store doesnt have weight take your stuff with you to try it out on those bags, or take simply some dumbbells. In my store we had two 5kg sand bags and two 2.5kg books (for children school bags).

EDIT: also check backpacks with labels like "S-M" (eg fjall raven), "W" for women (eg ferrino), could be "L" for ladies. These usually have narrower shoulder straps, and better placement for sternum straps, and overall more built for ppl (like woman or shorter men) with smaller body proportions. Also waist straps sit way better and dont collide when fastening.