When you are in the basic editor screen for a bib that has more than one 020, if you click the "Delete this tag" on the first 020, the save button stops working. I've attached a screen recording of me deleting the first 020 and then clicking the Save button as well as each of the options in the drop-down menu.
Created attachment 189955 [details] screen recording described in ticket
Rather than being about 020, that should be about how you have it marked as Important, which should be fixed when the patch for bug 40997 makes it to you.