SBC is incompliant with 3GPP TS 24.229 (SRVCC failure)

Hello community.

We have been struggling with SRVCC transfer failure for almost two years and finally have found out the reason of the problem. The call scenario is following:

1. Calling party (UE A) is VoLTE, called party (user B) is somewhere in PSTN network, actually it does not matter where user B is located. The call is in connected phase. Calling and called parties are talking.
2. Calling party keeps moving between mobile cells and at some point it losses LTE signal and performs CS fallback. VoLTE SBC, where ATCF session has been created , receives an indication of this event - INVITE with special STN-SR number that arrives from MGCF (from CS network). At this moment, from VoLTE SBC's perspective, calling party is in transition state from VoLTE to CS. Both legs - PS and CS are still active and anchored on VoLTE SBC.