[knot-dns-users] outgoing NOTIFYs over TCP

Hi! One of our customers uses Knot 2.6.7 as hidden master which sends NOTIFYs to our slave service. He reported that Knot can not send the NOTIFYs, ie: knotd[10808]: warning: [example.com.] notify, outgoing, 2a02:850:8::6@53: failed (connection reset) It seems that Knot sometimes tries to send the NOTIFY with TCP (I see also NOTIFYs via UDP). Unfortunatelly our NOTIFY-receiver only supports UDP. So, this is the first time seeing a name server sending NOTIFYs over TCP. Is this a typical behavior in Knot? Can I force Knot to send NOTIFYs always over UDP? Thanks Klaus