Why won't this conditional state_not prevent the card from showing?

Every now and then my Buienradar sensor has hiccups and because of the unconditioned weather-card, the entire weather view in which this card is configured won’t show.
Have seen that many a times, but never got to find out why. As it happens I was inspecting something else in Chrome yesterday, and noticed the error in the weather-card being ‘unknown’ .

I quickly made it into a conditional card, and the full view became live again, of course now without the weather card, because the condition was true (state was unknown).

Buienradar has returned, and now the condition apparently prevents the card from displaying stating:

 - type: conditional
    conditions:
      - entity: weather.woensdrecht
        state_not: 'unknown'
    card:
      - type: 'custom:weather-card-chart'
        title: Woensdrecht
        weather: weather.woensdrecht
        locale: nl

taking out the conditioner dn use the config again like the original I had:

  - type: 'custom:weather-card-chart'
    title: Woensdrecht
    weather: weather.woensdrecht
    locale: nl

reveals a nice chart:

why won’t the condition pass? weather.woensdrecht is not ‘unknown’ so should display, shouldn’t it?

using quotes or not around unknown doesn’t make a difference, tried that.
thanks for having a look!