I had another look at this because it was bugging me. I changed over one of the trunks from SIP to PJSIP and the transferred started working in one direction, you can transfer back to the SIP trunk from PJSIP. I then changed the other one and all was working, including showing the correct extension details.
So it would appear that it is the PBX sending the transfer back that fails and not the one receiving the call back?.
Unfortunately I only had one FPX13 the other is FBX12 so there are problems with it trying to register when using PJSIP therefore it may not be a fix I can leave in at the moment but as far as I can tell FBX13 to FBX13 should work fine if you use PJSIP on the trunks.
It is a pity SIP can not be fixed through.