Asterisk does not send CANCEL when calling a group

There are two numbers, 6020 and 6021. Two softphones on one computer, each of them is registered with its number. Dialing (SIP / 6020 & SIP / 6021,20 …) Calls come to both softphones, but when the handset is up (automatically) on the first and second, the second CANCEL does not arrive. Rather arrives, but very rarely. Why this can be? The analysis of the asterisk and WireShark logs did not show anything, the second one does not receive CANCEL and holds the call as if nothing had happened.
If make pause before answering a call from one of the phones, everything works as it should. Command CANCEL is comes.

I don’t think chan_sip is being actively maintained. Do you get the same problem on chan_pjsip?

chan_pjsip did not try. Basically SIP works as it should, except for the example that I brought.