It is not only an xperia problem.
I am running solus with same bluez version on my w541 and have the exact same situation.
For me only selected BT devices do work properly, for instance alpine car units and vintage nokia headsets work flawless with either xperia sfos and solus os for me.
On the other hand i got major problems with my collection of jbl bt devices. Non of them connects reliably or plays without stutter for more than 5 seconds. The situation is like that since release of bluez 5.6(?) somewhen last summer/fall imho.
Also, on solus i had slight improvement with os updates since then.
Most noteably, my wifes x230 with same solus version has and never had any of those problems.
From that i guess a problem with certain bt controllers and recent bluez verisons. But it is only my empirical observation and could be wrong.
Also only a feeling, but could it be depended on how close the (reception) device manufacturer follows the BT-Standard?
Maybe the devices making problems (JBL for me) are not clean implementations of the standard and require work arounds not implemented by small dev teams (Jolla/Solus)?