My first Fitbit (Flex - I believe) came apart like this....literally in 2 pieces!
I upgraded to a Fitbit Charge 2, thinking it would be better made....more expensive!!?? lol
It is doing the same thing!! I LOVE the Fitbit products - if they didn't fall apart so quickly!!
I bought the small, due to having small wrists and I actually wear mine in a more loose position thinking this would help with the stress on the band at the Fitbit/watch face.....Uh....it didn't help!!