Think I have figured it out...
Saved two test files.
1. Viewports of 3 references in the same place on top of one another. Saved with rotated plan and 'save referenced cache to disk' CHECKED. Re-open file and references are all in the same place and file is stable.
2. Same file but saved in rotated plan with 'save referenced cache to disk' UNCHECKED. Re-open file and references are artfully arranged in seemingly random positions and the file is unstable.
I had previously tried all of these other control tests:
? locating the information at the origin instead of at OS coordinates - no difference
? testing the process with other project data
? testing the process with each of the 3 references omitted
So I am therefore pretty certain that there is a bug which affects complex drawings (10k+ objects - ie. any relatively detailed GA architectural building plan or consultants drawing).
I would summarise the bug as such:
If a complex drawing is saved in rotated plan view without 'saving the cache to disk' then, upon reopening, the references behave strangely, appear in random locations and the drawing becomes corrupt. This is sometimes the case when a file is rotated and then returned to zero degrees before saving.
Problem is, the cached file is 7Mb, and the uncached file is 344k, so you can see the benefit of the uncached referencing for complex real-world referencing.
Has anyone else seen this problem or can anyone try and recreate?