Kamalio support for auth-int (and all md5-sess too) is likely to be broken. I've fixed quite few bugs originating from 2005 era in the scope of the RFC8760 Interop prep that we've done for OpenSIPIt few years ago. To the best of my knowledge none has been pulled back into Kamalio and when confronting Daniel about RFC8760 year or two ago he was not particularly interested.

Your best recourse therefore is disable auth-int on the OpenSIPS side while dealing with Kamalio.

-Max