I would like to clarify that the issue with local help is that Edge removes the query parameters when opening a local file programmatically. In other words, file:///<path>/index_csh.htm#944 becomes file:///<path>/index_csh.htm
We're also having this problem and have not found a solution yet.
I've moved your comment over into it's own thread because the thread you replied to concerned RH2017. What version are you running and is it up to date?
While not Robohelp, I found a post regarding the same problem using Flare. The principles would apply to Robohelp output I think, if you have a developer that can help out.
Or wait to see if the new chromium based edge fixes things: