I think it answers your/our question!
btw how does AROME handle the subject?
Yes, I think... but to clarify the "Freezing altitude" layer on Windy, should we understand that this level is counted as Altitude above sea level, but when this level intersects terrain located at a higher altitude , then this level is measured in relation to the ground itself, as Height above the ground.
So this layer should be called FREEZING LEVEL, as sometime it is measured as ALTITUDE above sea level (asl), sometime as HEIGHT above ground.
In Windy, if you check the Freezing level with AROME it switches to ECMWF. So this layer is not available with this model
In the example below (left map) the Freezing level is at 2400m in West of the Alps and at 0m just above the high summits, as described here above for ICON.
On same locations, the Freezing altitude from AROME data on Meteociel (Right map) is shown at 2700-2600m even when this altitude intersects mountains at 3000 or 4000m.
So there are different ways to display the Freezing level as we could try to summarise with this sketch:
I am more familiar with Freezing altitude asl, which is much more precise for outdoors activity in mountains. The Height above ground is too coarse due to the low altitude resolution of the orographic model integrated in the model itself.
Anyway, at least, it clarifies the way how this layer must be interpretated...
Thanks for your investigation