Grouping by optgroup loses items when searching

This can be reproduced on the optgroup demo page: https://crlcu.github.io/multiselect/examples/optgroup.html

1. Make sure all items are on the left side.
2. Notice under "German Cars" title there are 2 items: Audi and Mercedes
3. Search for Audi and move it to the right.
4. Clear the search field "Mercedes" and the grouping "German Cars" is gone.

To clarify, this only happens when using the optgroup. I don't know if there's an easy way to reset the remaining items so they all appear again? I've tried turning off the sorting but that didn't work.