I have Ur case too. No reasons are found on the decoded frames.
In the decoded signal that the MSC sent, this one support A5/1, A5/2 and A5/3. Our BSC support only two mode : A5/1 and A5/2.
In some cases, the MS send the ‘ Classmark Update’ before the ‘CM service Abort’. I decode one of theses cases and see that the MS support the 3 cipher mode.
For the information, no one have tried to deactivate ciphering.