I am having a problem with showing an items meta description. The meta description field is always using the value from the first menu item (not from global configuration). This happens even when I use protostar template and original k2 item.php. Joomla 3.8.1 and k2 2.8. Keyword field works fine, getting the value from the items keyword field. But not the description.

The items are linked to from a k2 category page, that also is the first main menu item, so it seems that the description-value from that category-page is joining over to the item-page.

Does that K2 Category directly belong to a menu item?

Yes, that is correct :-)

The menu item's description is being used in that case, if it is empty, then the fallback is the K2 category's.