In order to preserve display life, we had originally implemented an auto-dimming feature that kicked in after 30 seconds.  That does not seem to work in manual brightness mode like it used to.

It should so as to maximize product lifetime.  It may be that we don't actually want it to always go dimmer -- probably above 90% brightness, we'd like to auto-dim down to 90%, but below 90% there's probably no need to dim further.