The normal reason for a BYE immediately after ACK is that the codecs in the 200 OK were unacceptable. That might also extend to an unacceptable encryption offer. There is no way for 200 OK to be rejected, so the call has to be accepted and immediately closed.
127 is defined as “Internetworking Unspecified”, so it is a catch all, but might well cover that case.
With normal, early offer, SDP, the 200 OK should really be compatible with hte initial offer, but some systems (e.g. some Cisco ones) use late offer SDP, and the SDP on the 200 OK is the initial offer, so can’t take account of the initiator’s requirements.