I just saw this and thought my 3g RLC knowledge may give some hint..Not looked into LTE or 5g
If you see the 25.322 , it says that the
When "Poll" is configured, the Poll_SN field shall be set to VT(S)-1.
VT(S) - Send state variable.
This state variable contains the "Sequence Number" of the next AMD PDU to be transmitted for the first time (i.e. excluding retransmitted PDUs). It shall be updated after the aforementioned AMD PDU is transmitted or after transmission of a MRW SUFI which includes SN_MRWLENGTH >VT(S) (see subclause 11.6).
so I guess, POLL SN should always check the current next SN to be transmitted.
Just check, whether it is same for LTE and 5G..I guess the principle should be the same