Doka I can’t recall where I heard it but I remember from maybe 2 or 3 years ago a similar matter where the VLR, MSC, TAP couldn’t log the IMEI for a particular mobile.
Two problems were found:
1) the mobile sent its IMEI but coded in a format the network couldn’t understand. I think at that time particular areas of a network had not implemented updates to a number of MSCs/VLRs.
2) the other problem was the BS subsystem could forward the unknown IMEI format through the network as the format was unknown. This might be a reason why MSC/VLR and not seeing the IMEI.
This is just a suggestion as you mention the MSC/VLR are detecting other IMEIs of different makes/models
Recently two Nokias, N95 and 8800 Scirocco were found not to display IMEI following *#06#. Tests are being run to see whether the mobiles are transmitting IMEI.