I guess all I needed to do was report it here in order to figure it out!
How I fixed them: Set diagram color to red so that they'd be visible. Right-click the now-visible line to bring up a submenu. Under Format, "Use default" was clicked off. Click it on.
How they managed to become un-default, I don't know, but they're better now. I'll mark this answered but leave this here in case someone else stumbles into the same problem.