in 3GPP 24.008, it states that the MS shall wait for a response from the network (accept or reject) during T3210 (20s). If it is not received, then a second timer is triggered (T3211) for 15s.
At expiry, the Ms will restart a new location update.
The summary is at chapter “Timers for mobility management”.
What you’re saying is different though. It’s strange that the facto to be on SDCCH/8 or SDCCH/4 would change anything. The preiodicity of the SACCH is still the same (about 471ms), so that should not impact the radio link timeout.
When you do a trace with your drivetest tool, what are the layer 3 messages exchanged ?