Thise using evenflo wide mouth bottles/nipples are they supposed to “collapse” when baby’s eating?

I have Evenflo bottles but started with Dr. Brown’s when my baby was a newborn. Now that she’s 3.5 months old, I tried switching back to the Evenflo bottles since they hold more than 4 ounces.

I started with the original slow-flow nipple, but she kept falling asleep before finishing her feeds, which led to more nighttime waking. I switched to the medium-flow nipple, but with both sizes the nipple seems to flatten while she’s sucking. As soon as I take it out of her mouth, it returns to its shape as air goes back in.

I’m not sure if this is how the nipple is supposed to work since it’s designed to mimic the breast—maybe that compression is intentional. But I do think it might be slowing down her feeds, because she often leaves milk behind with the Evenflo bottles compared to the Dr. Brown’s Level 1 nipples she normally uses. Then again, she occasionally leaves an ounce or two with her usual bottles too, so I’m not sure if the nipple is the actual issue.