I'd be interested in reading a source for both of these points, if you have one. By "moving the Azure DevOps engineers to GitHub" do you mean they switched internal tooling on their own team? or that engineers from the ADO team switched orgs and now work for GitHub?
I can't find any info about anything in Github, much less actions specifically, being rewritten in c# either.
I feel like you're answering your own question.
Bad integration work, and/or bad architecture on one or both of Github and Azure's sides.