I don't tend to use the dynamic ucs because as all the lines are real life surveyed .. nothing is perfectly straight or on the same plane so this tends to give bad results and is just a pain.

The issue I now face is that going back to WCS I find that a lot of objects have different z values tied to various UCS rather than WCS. IE if you change the z value manually in properties or use FLATTEN in some cases the line will shoot off in the XY WCS direction.

I think the problem is caused by using polylines within the UCS as they are flat lines that only run along one elevation/Z and whatever blah blah programming,database etc etc it just doesn't work. So using things like FLATTEN can destroy your drawings (as I've been reminded of recently before saving a good backup -_-) I'm guessing this may be one of the reasons .

aaanyway I struggled to find an answer or the topic so thought I'd post this in case anyone else needs help 

BURST your blocks (to keep any ATTDEF info) and EXPLODE all of your 2d polylines  before using the FLATTEN command or touching the Z values.

and only ever use flatten on drawings you know were drawn completely in WCS or that don't contain 2dpolys.