Variable product dropdown not working after update

I’m being forced to change my variable products back to simple because the dropdown, which i use for quantity discounts, is not working. Everything is programmed fine and has been working for years.

After the update, when clicking the dropdown, it flashes the options really quickly then reverts to saying “choose option” but with no options, therefore people cannot add anything to the cart.
Whats going on? All template files say theyre up to date.