Descriptive Problem Summary:
See this post - appears to be a similar issue.

Numbered Steps to Reproduce Problem:
1. Open map.
2. Open icon file.
3. Add a new icon state to an icon file that is used on the map already (possibly by being set on the map directly rather than a default?)
4. Set a (new?) object to use the icon file and icon state.
5. Save the map.


Expected Results:
The map is saved.


Actual Results:
The map is saved, but in a corrupt state. Other objects that were using that icon file now have their icon as "!INVALID_VALUE!". The map is no longer able to be loaded in DM without manually repairing it.


Does the problem occur...
I have not been able to narrow down the exact steps.

When does the problem NOT occur?
The problem did not occur while saving the map previously; it was only when the icons were adjusted.


Workarounds:
Save early, save often (to a new file every time). Recovery may be possible by search-and-replacing instances of "!INVALID_VALUE!".