Setting 'sit on terrain' is not the problem (I have done some TG2 work before

), I think a distribution shader somehow 'throws dirt in the food'. I had it set at a minimum height, and at that height the trees indeed float, while the rest (upwards) is sitting alright. I'll try to make a less complicated tgd to see where it goes wrong. I sometimes manually drag the pop area to another location, and the Y would change then as well. But I cannot remember anything going wrong there.
A compute terrain at the end slows things down, I'd say, and shouldn't be necessary. But I'll try that.
And, no, it's not in the object.
Thanks for your comments, guys.