After testing, it is not clear to me what data in OpenStreetMap is evaluated with bike_network == MISSING?
Is it the RELATION with KEYS: cycle_network, route, type, network? For “network” most common types are rcn and lcn. I would presume route=bicycle. cycle_network is a text field contains the name of the network to which many routes may belong. Not all route=bicycle will necessarily belong to an official network, but are a signed route ie. cycle_network may be missing.
Is it using the highway TAG which can have many values including living_street, pedestrian, footway, path or cycleway. For any of these, the WAY may have the TAG bicycle=designated or bicycle=yes. Neither of these things say if the living_street, pedestrian, footway, path or cycleway is part of a route.
Which of this data is used by GraphHopper routing?