I just wanted to add that I’m also having this issue too.
I have a repeater field with a tabbed option in a dropdown.
I’ve even made the field NOT required, but it still fails on validation if you use the default option.
To make it validate, I need to individually go through each field and switch them to something else and then back again.
This is on a site with mutliple contributors, which is causing me all sorts of headaches. I can also confirm this only came about after updating from 5.1.8.
Cheers!
