Speed change doesn't work for Prores encoding

[Prince7]

When encoding using the "Speed change" filter, the "change speed" one does not work using Prores as a codec. Tried on all three Prores encoders.
"Scale output framerate" works however so the issue probably isn't the whole filter, and "change speed" does work properly when using the x264 codec.

Is this normal or is that a bug ? As using Shutter Encoder I am able to change the speed to a different framerate while encoding Prores, so this should be possible to do.Please, read the 'Infos needed to fix&reproduce bugs,..'-sticky before you post about a problem.