Now you led me to the interesting idea - PROLONG is activated whenever the target is lost (either velocity filter, or blocked line of sight or something) and it should mean that CVK is moving the RPC according to the computed target's track because the RPC can't see the target.

So I do not understand the PROLONG when AS-RPC is manually switched off and thus CVK can not control position of RPC. In this situation in my opinion the PROLONG should not be activated....