I create a document library and a custom list, then use the custom list as the source for a lookup column in the library. Now I add a document to the library and set the value of the lookup column. Check version history and we can see version 0.1 ( adding doc ) and then version 0.2 ( setting the value in the lookup column )... - all good so far
Now switch back to the custom list and modify the same value chosen in the library item.
Switch back to library and we can see that the lookup value has changed - as expected - check version history and;
The version still shows last modified as version 0.2 where the property was set - but now shows the new value from the lookup.
So the question is how do you make the version history recognise that an underlying lookup value has changed and create a new version in the version history to reflect that change?
I'm hoping there is just a check box somewhere I missed.