I note your BSC is ALU and your MSC is Ericsson, therefore I deduce there ‘maybe’ an issue with inconsistent parameter settings.
Typically in an Ericsson network, the T3212 parameter should be set relative to MSC parameter BTDM (Base Time Duration), which results in detach.
BTDM is the time supervision for implicit detach and it is crucial for BTDM to be higher than T3212 (i.e. BTDM ≥ T3212) so that the MS is not detach from the network before the period registration kicks in.
If BTDM is less than T3212 (BTDM < T3212), this will result in the MS getting detached earlier than MS doing periodic location update. The typical default values I have seen for T3212 is 30 or 40 (3 – 4 hours), with 40 being the normal default. In Ericsson the units of T3212 and BTDM (for Ericsson MSC) are different as well, i.e. T3212 uses deci-hour and BTDB uses minutes. I am unsure what the settings are in your ALU BSC, however please ensure your T3212 timer in ALU BSC is less than or equal to the BTDM in your Ericsson MSC.