If you select multiple fixtures that have different Z values across ladders and stands and whatnot and move them in any way (nudge, [CTR M], drag and drop, etc), they all take on the same Z value. For instance, if you've just re-spaced, re-trimmed and jiggered your 5 trusses full of lights until your new rig is just about perfect (multi-rung ladders included to make it all the more frustrating) then grab them all and move them a few feet upstage, you will discover that all of the ladder fixtures are now at the same height of the closest truss they were near. Its not consistent - you can move them a few times just fine then all of a sudden you blink twice, change your view and look again: all the ladder fixtures have jumped to the truss height.
If you thought you might like to take that whole arrangement, maybe even as a group and move them into a new document, let's say one with a venue, scenic, seating and audio layout in it already? Don't bother unless you want to re-trim all your lights, again... This one is consistent - copy from one doc, paste to another - bye bye ladders! (see screenshots) Anyone else have the same thing happen? Is this documented somewhere?