Miguel, I saved both versions of your route in one folder so that I could look at them both side by side.

It appears that the location of certain viapoints (6&7) is causing the increase in distance due to a detour to reach VP 6:

This kind of thing is common if we change 'styles' after adding viapoints.

Also, when "paved" is select, it is impossible to make it going on the route I want (like drawn), even though there is no unpaved segments there.

Have you tried starting from the beginning, having selected 'paved'?

If so, I'm guessing there is something in the underlying OSM data that is tagging the relevant part as unpaved. If you're confident that the section is rideable, it is possible to 'mix' styles as outlined in this thread.  

Hi Hobbes,

There should be something in the Osm data. Look here, without waypoints https://cycle.travel/map/journey/760702 it just does the tour around. The road is good paved (I did already) and is part of a national bike route.

Hi,

Just found out that just next to the end point of the route I gave there is in Osm few meters of cobblestone. I just passed there today and I can confirm. 

@Richard, I thought last year you had put some exceptions for short segments of unpaved to be ignored in the 'paved' road routes. In Switzerland there are many of these. Here is just an example. 

Regards, 

Miguel Anjo 

The route planner’s core algorithm doesn’t really lend itself to ignoring short sections sadly, but cobbles are a bit of a special case. I suspect here it’s the “unhewn cobblestone” that’s throwing it. I’ll take a look.