220.127.116.11575 (paired it back to my hue bridge temporarily to ensure the latest firmware is on board)
Spent the last 20 minutes testing this on the above firmware (which is the latest available - dating back to March 2019).
(1) Same result - illuminance mostly (but not always - maybe about 90+% of the time) updates on a lux threshold change when the motion sensor goes active.
(2) While the motion sensor remains in its active state, illumination changes are also mostly (but not always) reported as soon as a change occurs
(3) while the motion sensor is inactive - nothing happens on a change in illumination beyond the threshold setting (in either direction). I assume it will get reported at some point (eg when there is a temperature report). I'll leave it alone somewhere over the next day to see what happens.
Only reported this now as I had not taken any notice of the illumination reading until I started messing with the motion lighting app using this as a condition.
EDIT: For case 3, it seems to update for a change in illuminance at a minimum interval of 5 minutes (when set to 1 lux change reporting). When set to report at the same time as temperature we get nothing at all. I assume the threshold testing is done at the end of each 5 minute interval.
From the motion lighting app perspective I suggest that we can use the device driver behaviour to test for illumination when motion is triggered rather than use 5 minute old data. Hopefully the team can take this on as an option in the app. Any chance of this @mike.maxwell?