Asset import anomaly (with workaround) upgrading project from 5.5.x

I had tried the 5.6 beta a couple of months ago (with a copy of my project) but had to drop it when I found that some of my critical purchased tools weren't yet 5.6-ready. Yesterday I revisited 5.6, and my project now imports and runs nicely thanks to updates from asset providers.

During the import of my project (again, a copy for testing!) from 5.5.2f1, I found an interesting anomaly. I will not call it a "bug", but just something that perhaps should be documented. I also have an easy workaround.

Many of my mesh assets were imported from Blender, and to aid that process I have a custom C# script that takes care of the axis reorientation (Blender thinks Z is up, and unity thinks Y is up). My script handles the situation of nested parent/child objects from Blender better than Blender's own FBX export does.

When I imported, there were no console errors, but all my Blender imports were disoriented because the axes reverted during Unity 5.6 import.

Remembering my custom script, I initially theorized that perhaps the AssetPostprocessor.OnPostprocessModel() API had been renamed or deprecated, and my script was no longer being invoked. But I checked, and this was not the situation.

Next, I decided to try manually reimporting one of my Blender models from the Project view, to see if there was a useful error in the console. When I did this, the asset reimported with correct orientation, demonstrating that my script still works fine. Rinse and repeat for my other FBX models, and all is well.

Hypothesis: I think that during the upgrade, Unity may have imported my FBX assets before it compiled my C# post processing script. This would explain why the problem is corrected by reimporting.

I really don't think this is a Unity bug, but I wanted to post the behavior in case someone else encounters this situation. If the Unity team thinks my hypothesis for why it happened is reasonable, perhaps you could add a comment in the upgrade instructions to warn developers that they may need to manually reimport some assets if custom post-processing scripts were in use.