Markdown preview does not work properly when the nested list item is a tab. Is this expected behavior?

Should work with both spaces or tab indents
It is a bug from Remark markdown parser. Check this issue. https://github.com/remarkjs/remark/issues/198 Once the bug is fixed, we could apply the fix to the app too.
Until this issue is fixed, please use spaces.
There might be another angle to this. When Preferences/Editor/Editor Indent Type is set to spaces it still inserts tabs. If it inserted spaces as it should, I believe that the lists would work. #483.
BoostNote 0.5.0.
Cheers.
Apparently, the issue in remarkjs has been fixed. So I'll try to fix this in v0.9.0.