I think I found the root of the problem. The custom list is automatically populated by category. The problem is that it populates automatically when the podcast has no category at all. The odd thing is that sometimes it populates randomly with all the downloads. I'm guessing there's a gitch somewhere preventing the app to determine the category. It happened to me with a particular podcast, all its episodes went to the custom list. I'd have to check but I'm sure it has no category set. Today, all my news went to the custom list and they were in the -- list, empty category I guess. I deleted them and downloaded again, they went all to the right categories. One thing that can be done to solve this is not to add episodes with no category to the custom list and add a selection to the category list, something like "disabled" or "none" to show the list empty. I tried deleting all categories and it doesn't work. Now I have the list stuck with News because it was the last selection I did to test the problem. It's not a big deal, really. Just a little annoying. I think that the problem is that, after being populated automatically, the playlist goes automatically to custom. I'm not sure and now I have no way to test it, but based on the description of the comments and what happened to me today, I think that's the case. If not, nobody would have noticed the custom list.

@Anonymous
Please send me an email in case you have a question / issue so I can help (Settings/Feedback)
You can do this enabling Settings/Playlist/Skip played content.
You can also disable streaming from the playlist settings

Is there a way to add ONLY unplayed / downloaded episodes of a certain category to a custom list. Currently when I create a custom list all the episodes get added even the ones I already listed too. I did not see this feature in the advanced filter options.

@Justin Categories are set, custom lists would be created by the user for personal use.

How is that different than categories? I don't quite understand