cycle.travel's scoring for the two routes differs by 0.4% so it's very close. In OSM data terms, the challenge is that there's no particular special tagging for the Sarphatistraat - it just gets classed as a "cycleway", same as the route along Mauritskade. Plus there's a couple of very slight advantages for the Panamalaan (which is tagged as part of a designated longer-distance bike route) and the Zeeburgerdijk/Mauritskade (which are beside water so get a scenic uplift).
I might look at doing some Amsterdam-specific routing tweaks: I doubt the longer-distance bike route consideration should be relevant in this case, for example.
> I doubt the longer-distance bike route consideration should be relevant in this case
Or maybe it is, but then it's relevant for all routes involved, including the Sarphatistraat (which is short distance for cars, but long distance for bikes).
Try "Feike de Boerlaan" to "Vondelpark". I would expect the trip to take the Czaar Peterstraat and then the Sarphatistraat, but instead it takes the Panamalaan, Zeeburgerdijk, Mauritskade, and only switches to the Sarphatistraat at the Weesperplein, which I think is also the point where the Sarphatistraat changes from a bike street to a regular street with a bike lane. Although it later switches back to bike street again yet the route stays on the bike street.
So I have no idea what's going on with that first part of the Sarphatistraat. It's a really nice route, yet all apps hate it.