are you using EGSM or just PGSM in your BBH cell ?
are you using compressed inter RAT HO info ?
which HSN are you using ?
which MAIOs ?
in the immediate assign messsgae, only those few “important” info are sent… so only one of those is causing the problem.
for hopping, the cell is also sending its time synchronization info : T1, T2, T3, but it is not configureable (i think). Thanks to this sync-info, the MS is able to guess what is the hopping sequence.