For the past few versions of the Mac Poser 11.3, I have trouble picking a part (forearm, hand, etc.) with the mouse, and then when I do pick correctly, or specify the part with the pull-down menus, the Parameter panel does not change to the right part. It usually displays Parameter dials for "Ground", or "Camera". This makes posing via dials hard, obviously. I have not found a way of fixing this, other than to write the figure, restart Poser and read the figure back. After restarting, the correct Parameter panel appears.

Anybody else having this trouble? Is it a bug, or is there some feature I am not aware of?

This is a bug that has managed to evade its due squashing for quite some time. Like you said, restarting Poser is a workaround. Though I've found a lot of times you can get the properties panel to refresh with the correct parameters if press the numbers 1, 2, 3 a few times. (the numbers above the letters/under the f keys...not the num pad on the right) 1 selects element (body parts/props) 2 selects figure 3 selects camera so try selecting the body part then hitting 2, 3, 1. If that doesn't work then def restarts. Its annoying but ive got around this for ages. (hope they fix it one day)