location update reject is not a sdcch drop, unless there is a problem with your NSS. You see, the location update is not acknowledged, but the radio link itself was established and released succesfully.
To clear the matter:
1/ during drivetest or with qos statistics, do you see good or poor radio conditions (call drop rate, ho quality, ho interference, AMR usage, radiolink timeout “S” counter, etc.)
if yes -> interference. Activate “SFH” 1×1, put almost all your frequency band onto those TRXs, and locate your SDCCH timeslots on “non-bcch” TRX.
Check also that your NCC (network colour code) is not the same as your neighbor country.
if no -> then it might be a signalling problem (even though it’s not logical…)
Do you have possibility to do an A-interface trace or Abis interface on those cells ? You will see if there is a problem during this signalling phase.