I've not used SyncSort with JOINKEYS. The way DFSORT works the SORT is automatic, on the keys. If you can't specify SORT in the JNFnCNTL, then it looks like SyncSort is the same.
So, why was the output you showed not SORTed? Plan B is that you don't have the correct start positions. You also have the RESTART on the same positions, and if you look at the output you showed the AAAA get number 1, 1, 2 and should have been 1, 2, 3.
Is your file an FB, or a VB? (I was hoping to see on the sysout).
//SYSIN DD *
//JNF1CNTL DD *
//JNF2CNTL DD *
Try with that, and show the full input records, and the full output records. Check your start positions. Show the full sysout including all the message numbers please.