Since 5.6.0 every dropdown in an element does not work any more. There might be a js error, stopping the dropdown from working. It is not a specific one, but every one.
I was having the same issue. I did a hard refresh on a wp-admin page and all is working again.
I can’t tell by looking at this what type of field it is. “drop down” could mean any number of things. Is it a “select” field or some other type of field.
I’m not seeing any errors in my fields. I would first use @pattyok’s advice and make sure you don’t have a browser caching issue.
It also looks like you have some custom CSS and maybe some custom JS by the look of the fields? If this is the case maybe that’s the issue. Also try deactivating other plugins and maybe switching to a default theme to see if you can narrow down where the conflict might be.