I need to ask which jbl model(s) you have.

Mine are Flip 3 and Charge 2 from late 2016.
Both work fine with any other bluetooth a2dp capable device i own and tested from friends.
Also my beloved alpine car head unit shows the same symptoms as the jbls with sailfish x.

Connection works.
But after 10-30 seconds of actual playback, some severe stuttering occurs for some seconds.
Followed by seconds of double speed playback.
It feels like a buffer underrun followed by a try to catch up to sync.

Another observation is, my solus 3.9999 linux lenovo w541 laptop behaves the same as sailfish X with those 3 bt devices

Sailfish x and solus OS utilize the same Bluez version so my hope is that some fix might come from that side "somewhen".