Sounds distinctly Boeing to me. On the B777, the FMC transitions to "on approach" under the following conditions:
- the aircraft is in the descent phase and the flaps are out of up, or
- a published approach has been selected in the active flight plan, and
- the aircraft has sequenced the first waypoint of the published approach, or
- the aircraft is enroute to a direct-to or intercept-to waypoint and is within 12nm of the runway.
If VNAV speed intervention is being used and the FMC transitions to "on approach", the speed window will close and the FMC will command whatever speed was programmed at that point in the approach profile; it may be 250 knots if further out, or whatever speed constraint was programmed at the FAF if close in. There's no way around it, other than changing the speed in the FMC.
When the FMC transitions to "on approach", the IAS/MACH window can be opened, and the command speed can be set while VNAV remains in VNAV PTH. Once the window is open, VNAV will command the speed set in the window.
Boeing recommends using VNAV with speed intervention active (ie the speed window open) during the final approach. They don't recommend adding extra speed constraints to the final approach waypoints.