Flat mode for bicycle is not flat

When using bicycle only mode, the flat mode on triangle don't seems to work as expected. The itinerary suggested if far to be the flattest, and even the "quick" mode show a flattest path, which doesn't make sense.

Expected behavior

The "flat" mode should find a flattest path, and at least not less than the "quick" mode.

Observed behavior

"Flat" mode isn't the flattest (there is some flattest path for this journey), and is even less flat than "quick" mode.

Version of OTP used (exact commit hash or JAR name)

docker.io/opentripplanner/opentripplanner:2.5.0

Data sets in use (links to GTFS and OSM PBF files)

Belgium pbf: https://download.geofabrik.de/europe/belgium-latest.osm.pbf
Belgium elevation: https://publish.geo.be/geonetwork/d51d2fe3-4309-4a1b-a7cd-28d8f60c9dc1/eng/catalog.search#/metadata/6657e6da-7345-416f-bef6-c6a8b2def9bd
Some Belgian GTFS, but probably irrelevant here.

Command line used to start OTP

services:
  opentripplanner:
    image: docker.io/opentripplanner/opentripplanner:2.5.0
    command: --load --serve

Router config and graph build config JSON

// otp-config.json
{
  "otpFeatures": {
    "FaresV2": true
  }
}

// router-config.json
{
  "routingDefaults": {
    "numItineraries": 12,
    "walk": {
      "speed": 1.7,
      "reluctance": 1.5
    },
    "waitReluctance": 1.5
  },
  "updaters": [
    ... // Probably irrelevant here
  ]
}

Steps to reproduce the problem

Query a "bicycle only" path, and use "100% flat" mode.

opentripplanner / OpenTripPlanner