sorry to give an outlier data point but i've had some sort of claustrum case since my iphone 5, 6, 7 and now iphone 8. i haven't noticed any irregularities in reception during that time. i also regularly use airpods and their signal doesn't seem to be diminished, although the phone is usually either in my pocket or in a bag on my body so they're never that far away.
definitely an interesting observation though, but in talking with the designer he made a conscious effort to not block the antenna seams on the phone.
which version of iphone do you have? (sim free, or carrier?)