What this tells me is that on the newer tractors, at least in this soze range, the Blk be bodies that split up the fluid for the steering and the power beyond (or the loader), seems to give more priority to the loader under load, starving the power steering circuit of fluid or pressure.
I know, it doesn’t sound right, but when it gets down to holding up a load or a situation where you need to turn the wheel while not moving, the load gets the fluid and pressure.
I have not tried pulling the refs up to see if I still experience that when the loader is under pressure to perform. It’s possible that it’s just that one can’t do many things at once when the tractor is in more of a stand still position, possibly under a lower general RPM than when moving.
The HSTC Grand L has throttle advance, which allows the engine to idle when your not working and then throttle up when yiu hit the pedal. So at idle, under load at the bucket, there isn’t a lot of fluid pressure or movement going on under the hood. I suspect that was what was causing the exact same thing you experienced, to happen to me.