For me this is not a drop.
This counter works when you receive on rach request with valid cause, you assign a SDCCH, no MS appear on channel in certain time (i don’t remember the exact timer value), and the channel is released with this cause. This counter is on ABIS that’s why they called it Abis fail.
No one could tell if this is really a MS. Also why drop if you never get on the channel.
Look at Nokia explanation
Phantoms affect SDCCH_abis_fail_*
SDCCH drop ratio counts the ratio of all SDCCH failures to SDCCH seizures.
Normally most of the seizures are caused because of phantoms which are counted as SDCCH_abis_fail_call and SDCCH_abis_fail_old. In practice, the latter case does not practically occur because SDCCH handovers are usually not
used and particularly because phantoms do not perform handover.
The percentage of SDCCH_abis_fail_call used to be very high in a low traffic
network (even tens of per cent) whereas in a high traffic network the percentage settled down to around 18-20 per cent. In BTS B9, BTS RACH detection was improved, and figures well under 10 per cent are now typical.