Issue disappeared after the micro 3G site right in front of my problematic cell was swapped –> as soon as it was put in the same RNC, the FACH drop disappeared.
The root issue wasn’t caused by FACH load, as I supposed few weeks ago. Drops occured because it isn’t possible to do a inter-RNC SHO when in FACH. The FACH is supposed to be dropped before the UE moves to the inter-RNC neighbour. It’s a normal behaviour (at least in ericsson).