I checked with a guru. He emailed me this:

"This appears to be a bug or a feature that only works when the user has

total control of the reference files and the active design file during an

editing session. I consider it a feature when the user is in full control of

the active design file and reference file. For instance, if an associative

region is created from reference elements, that region will remain

associated if the reference file is activated, then edited during the

current design session. If the reference file has been modified externally

by someone else, and then the original design file with the associative

region is opened, the associative shape fails and becomes dashed. I consider

it a possible bug because I have not found any means to re-associate the

region/shape to its original elements. In addition, I find no way to remove

or update the dashed symbology of the failed region outline. Even when

dropping the elements associative property.

I would recommend that the shapes are not placed with an associative

boundary to referenced elements. Only use this feature with active elements.

As for converting failed elements back to their original state; I would

recommend moving the failed element to a blank area in your design file,

dropping the complex status until you are left with just the boundary

elements, (lines, arcs, etc..), then use the create region tool to create a

new element with the flood tool. Once you have the new element, place it

back in its proper location and delete the dropped elements."