Atto,
How amazing… it’s impossible that the MS listen to the AGCH channel if it didn’t initiate it with a Channel Request.
(for info) Channel request is a layer 3 msg, you should see it just before the I.A. msg.
What is the content of the I.A. message ?
Can you check that the MS is in DRX mode while on the BCCH ? (discontinuous reception)