Grouping by optgroup loses items when searching

[ghost]

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.

[ghost]

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.