Menu not saving sub-items

A few hours ago, the menu feature (the standard one that comes with all installations) began malfunctioning. I have been trying to save sub-items in my main navigation menu, but every time I click “Save Menu”, the items become un-nested. Also, it will not let me even attempt nesting sub-items.

The ideal menu would look something like this:
Link #1
-Sub-item#1
-Sub-item#2
–sub item for sub-item#2
Link #2
Link #3

The closest I can possibly get before saving the menu is this:
Link #1
-Sub-item#1
-Sub-item#2
-sub item for sub-item#2
Link #2
Link #3

And, after I save the menu, it becomes this:
Link #1
Sub-item#1
Sub-item#2
sub item for sub-item#2
Link #2
Link #3

This is supremely frustrating, because this was, in fact, working earlier today (around 7am EST). I have restarted, rebooted, reinstalled, updated, etc. and nothing is working. I have switched themes (ideally, I’m using Salient. However, I know that menus are part of the WordPress core, so I have tried using 2014 and 2013 with no avail), turned off all plugins, and tried it on multiple local installations.

I am out of ideas. Please help!

https://wordpress.org/plugins/menu/