Similar issue brought up in “Auto delete old unplayed episodes” by @jbohn - I am not alone!
I have a far-from-perfect workaround.The podcast is an hourly news bulletin, which I normally listen to once a day at a specific time (ie when I take the dog out at noon!). Using “Episode Filter” I have only the episode with that time in the title downloaded. Works fine except if fills up “New Episodes” with the non-downloaded ones - and on the days I take the dog earlier or later!