Birkenstock Bostons - Feel like they are always going to come off

Hi, just got a pair of oiled black bostons with supergrip sole. Nice shoes but I find that my toes tend to try to curl and grip the bottom when I'm walking because it seems like they are going to come off. They were properly sized and I have the straps pulled all the way. Does this ever improve as I break them in or will it always feel like they may come off?

Any tips? Thanks