Dear, CM! I am interesting in your previous research for UL assignment about 2 months ago:) Did you got success in it?
Concerning your current question, I am thinking the following:
MS changes its state from packet transfer to packet idle right after receiving Packet UL ACK (with RRBP valid and FBI=1) and sending PACKET CONTROL ACK.
In turn Packet UL ACK (with RRBP valid and FBI=1) is sent to MS after receiving the last RLC/MAC block with CV=0.
The description above is about non-extended UL TBF.
In extended UL TBF mode the network does as Pix already said).
There is a non-standard (vendor dependent) timer on network side (for that extended case) which supervises the instant of time when Packet UL ACK (with FBI=1) must be sent.