The attached script does update my bind9 instance but reports SERVFAIL for Knot.
That would point towards your knot config. Perhaps if you share it (sans keys of course) someone might spot something.
I have DDNS working with knot v1 so I can compare configs if you happen to still be on v1. I also posted something a few months back about the problems I was having and what I did to get it going which may help. I don’t know how much this applies to v2 as I haven’t got there yet.
I am sending updates by calling knsupdate from a shell script.
Andrew