Then I want the steps to reproduce them. And this is not because I don't believe you, but because I have not been able to induce what you're describing.
I have never had any search engine "silently changed" to anything with the exception of once during one of the major updates, I think to version 1607, where a bunch of other settings that users can reset got set back to the defaults that Microsoft thought appropriate. I have expressed my outrage at this practice, or at least not telling people that they need to recheck certain specific settings, repeatedly.
I just clicked on a link in a Help dialog in MS-Publisher 2010 and it's being presented in Firefox, my default browser. If I can replicate what you're reporting then I may also be able to figure out how to block it from occurring. The only thing I've come to accept that I have no control over is what happens with Cortana web search results (always shown in MS-Edge and always produced using Bing Search).
Presenting the willfully ignorant with facts is the very definition of casting pearls before swine.