Importing version 2 scores into version 3
It seems there is something inherently broken when importing version 2 scores into version 3. Editing or adding several items imported act different if the same item is added to a score created in version 3. Some of this has to do with things like the continuation text missing from (de)crescendos, but others have no logical reason, unless they are related to the text issues Marc was working on. Most of the strange behavior has to do with text rather than notes, slurs and lines.
Comments
The main thing I know that is broken has to do with extra clefs appearing. There are several related issues reported about that. Plus the issue you reported recently about the spatiumdependent flag not being honored for some partocular imported score, although it works fine for one I created from scratch in 2.3.2., so I'm not totally sure what made yours fail. The lack of ability to set continue text on lines isn't just in imported scores, needs to get added to the Inspector. I think the issue you submitted probbly covers that already.
Other than these, you should probably submit specific issues for other specific things you see going wrong. Feel free to do a search to see if they've already been reported, but frankly, text stuff has changed so much of late that existing issues more than a week or two old may not apply any more anyhow.