I posted this in your owning/operating thread as well:
1) When you have the tractor in lowest range, be it a gear tractor or HST, and you demand maximum forward power but prevent movement (against a large tree, or tied to something) one of two things will happen, right? The tractor will either A) spin one or more tires, or B)if your traction exceeds your power output the tractor will stall. If you are able to stall the tractor than your traction exceeds your power output. This will happen with both an HST and gear tractor, and I don't see this as a problem at all.
Many of the higher horsepower per pound tractors (JD, Kubota, etc.) have more power than traction in low range so they will instead spin the tires in the same condition.
If you had said the tractor will not spin its tires and the engine will not stall, then that would mean the relief valve in the HST has met its maximum power output. That could be normal operation or potentially point to a relief valve out-of-spec. But since the tractor stalls this does not point to a problem with the transmission. It is possible that the engine is not making it's rated power, the best way to test would be to find a tractor dynomometer. The higher output (CT 230, CT 235) may spin its tires in this same condition, but then again it could exceed the power rating of the HST and engage the relief valve.
2) What does the engine do in this condition? Does it (nearly) maintain RPM or does it stall? If it stalls, again you have more traction than power. If the engine maintains RPM the HST relief valve has engaged which may or may not be as designed. Although, 16-20 degrees is significant. You may simply need to use low range in this condition. It's possible that a gear tractor may be better suited for this condition because it will likely have at least one range between "low" and "mid" so you could travel a bit faster, though it still wouldn't be easy to begin from a stop while using the clutch...