When a submodule is changed, and when clicking on it in the staging list, the diff is not shown anymore, so it's impossible to know to what commit it was pointing before, and to what commit it's pointing now.

This was working in earlier versions of SourceTree, before updating to 1.9.2. Now it says "No preview available".
It looks like this is only happening when there is a conflict on the submodule commit to choose, which is exactly when you need it the most!