PDF Forms: Combo Box Limitations?

I've just got started with PDF forms. I've designed a form in InDesign CS6 including a Combobox with 21 items.
I export to PDF just fine.

In Acrobat:
When I click on the combobox the list appears but only the first 17 items are shown.

If I click and item and use the down arrow I can get to the bottom, missing 4 items.

If I go to edit the form field, using properties and the options tab all 21 items are listed.
If I reorder them they are still limited to the first 17 items.

Is this normal?

Is it possible to have a combo box with more than 17 items?

Don't mind using a bit of Javascript.

There is plenty of space for the dropdown to appear.