Great details, thanks ! There is one inconsistency in your post, so let’s clarify:
What is the reason of the drop ? You are saying the BTS doesn’t send measurement report anymore… but the BTS never sent measurement reports, ever. Those meas reports are UL messages, sent by the MS to the BTS.
Do you mean the MS does not receive the DL SACCH blocks, therefore its Radio Link Timeout reaches 0, provoking a radio drop ? The MS does not send any message to the BTS, prior to the drop ?
the release indication is sent BY the MS ? that looks like this is the correct behaviour: at expiry of T3210, the connection is released.
So the problem is with the mainBCCH, you got to find the reason of the drop… If the BTS does not send DL SACCH anymore, it’s not normal at all. I can’t imagine why the BTS would not send those SACCH… but you can verify this on layer 3. You can also check the value of the Radio Link Counter “S”, it should appear in the drivetest as well.