Converting from kepub to epub loses ToC hierarchy

When I convert a kepub to a epub, the ToC gets 'flattened'
Where I normally would have
-Part 1
-- Chapter 1
-- Chapter 2
-Part 2
-- Chapter 3
-- Chapter 4

After conversion I get

- Part 1
- Chapter 1
- Chapter 2
- Part 2
- Chapter 3
- Chapter 4

and so on.

Is this an issue with kepub/epub conversion or am I missing some settings?