I was attempting routing from Scunthorpe to Grimsby by bike, and Open Route Service sent me on a bizarre 200 mile route via Nottingham so I've gone over through the area looking for anything obviously wrong. I've fixed a few bits and pieces (mostly cosmetic), but it all looked routable so I've no idea what the problem is.
The entire area needs work, not just Grimsby (which was recently flagged as the worst mapped place in the UK). It's relatively flat so mapping by bike should be fairly quick, but there's a large area to cover and existing data to check (you know when you get the "that's got to be wrong" feeling). There are existing gpx plots, some of which I have added roads alongside today but others don't match up to existing data very well.
I might be tempted to spend a week camping in the area doing lots of cycling/mapping in a few months when it's warm enough.
Edit: Upon further investigation, Open Route Service appears to try and avoid highway=unclassified when routing by bike or foot. Or at least that's what it looks like is happening in this part of the country. Can't think of any reason why highway=unclassified should be avoided though, surely it would be more preferable to a busy main road for walkers and cyclists?