When you copy or duplicate a default code, it used to place it above or below the original code you copied or duplicated. That behavior has disappeared and it's a very vital timesaver so we don't have to open our custom CSS files to find what we just copied in order to edit it.
I need to clarify something as I hadn't thought about it when I was writing before. This issue is only when using the dots to the right's menu. When you use the Create button setup that does what it should just fine.
This is also only when you're on the Styles tab. If you have a custom CSS file open and you duplicate in there the duplicated class shows up fine. Only in the Styles tab if you duplicate or copy it doesn't show up at all.
Another point on this (sorry I keep finding things that are relevant as I'm working). When you click somewhere else on the page, or in the HTML window etc. then click back, the new classes then show up in the Styles window, so they are there (which I already knew since I had to open the custom CSS file to see them prior to realizing I could just click outside and back again). They are just not showing up on creation like they should be and a they were doing before.
Thank you for reporting this! It is indeed a bug. We made changes to the Styles panel and this is a case we've missed. The styles are there, you can see them by switching to a different HTML tag and then back, but the list is not refreshed. We will fix this in the upcoming update.
Thanks Martin!