I know this is another apples/oranges comment, but on my old VW diesels (I had 3 of them), I could idle that sucker all day long and never get a whole lot of heat, but once I had it running a minute or two and headed down the road, only THEN could I get heat out of it. Something about it being thermally efficient or some such. On the coldest days I'd chuck a small pc of cardboard in front of the rad, so I wouldn't freeze my butt off on my 35minute commute to work.
My MINI COOPER (gas) is much the same, unless I work it, I get very very little heat from it.
If it were me, I'd yank the thermostat and put it in a pot of boiling water and see what temp it opens at.
Second thing I'd do, if that was OK, would be the cardboard on the rad thing.
YYMV and I have very little tractor experience, but been playing with cars and bikes for nearly 50yrs.