In the meantime I’ve managed to get what I needed by using WaveTable envelope with a Linear (Ramp In) wavetable and with mode set to “unit” (so it ends at 1.0), and then implemented the curves on my own (by copying from SuperCollider’s ExpWarp class):
Linear:
Exponential:
With this approach “start” can be larger or smaller than “end” and it works anyway (going up or down).
Still the behaviour of Evaluate WaveTable completely baffles me.