My guess is that one or more teeth on your height adjustment knob/cog is broken. My mind is drawing a blank as to the exact name for the cog but I have attached a photo for reference. If you remove the mower deck, raise the 3pt, and play with the height adjustment knob while looking on the underside you will immediately see what I'm talking about. One of the teeth on mine is broken and the the deck drops lower than it should. Adjusting the height knob either one step higher or lower is ok. I haven't got around to replacing mine yet as I still need to spend a bit of time assessing how to disassemble to replace most efficiently.
If you adjust the knob one or two steps away does it behavior properly?
If your machine is new, your height knob may not be aligned properly. E.g. the max height is really setting it to the install position instead of max height/install. It may be as simple as a misaligned decal. This doesn't sound like what you are describing, and if this is a developed problem, then obviously this wouldn't apply.
Good luck