It seems that IFS takes into account the altitude of the location which is not the case of ICON-EU. Look the two following screenshots for the same location which is roughly at 500m, at the same moment :

ISO 0°C with ECMWF (IFS)

ISO 0°C with ICON-EU

As you can easily see, the delta between the 2 values is 500m so the exact altitude of the location. If you choose other locations, you will see the same behaviour. There is always a delta fitting more or less with the altitude of the location, and not because the models are giving two different values,...

This means that the data of the 2 maps are not coming from the same calculation. IFS is given the altitude of the isotherm 0°C using the location as a reference so here 200m above Hockai (200+500=700m) while ICON-EU seems to give the altitude of the isotherm with the sea level as a reference.