miconda left a comment (kamailio/kamailio#4268)
There was a commit to send BYE requests on both sides always (2b0b8714b7f75d13be6f8156f4f8efbf6401f0b7), but in your case I understand that Kamailio sends BYE to caller and the callee sends BYE to Kamailio immediately, before Kamailio is sending the BYE to callee?