The discrepancy between half throttle and idle warmups is the DPF. Idling fills the DPF faster than any other RPM; full throttle fills it the least. Kubota picked the mid point between too much engine wear when cold and too much filling of the DPF.
The B-series (aside from the 3350) all use indirect, mechanical injection and no computer. They have no DPF, no EGR, no electronic control over the engine at all, other than the starter and electric fuel solenoid to shut it off. They claim "Tier 4 Final compliant," which they are, but the reality is that all diesel engines 26HP and under simply are not emissions controlled.
My 2018 MX4800 says to start the engine with the throttle at 50%. This throttle is 100% electronic. The engine will start at idle and about 1 or 2 seconds later increase the RPM to 50%. I start my 2013 B2920 at idle.
My MX probably saw about 2 hours on the clock at lower RPM before I ran it almost always at wide-open throttle for the next 60 hours which is where it stands now. I don't expect any harm to come to the engine.