I also use Feeder, and have been using it for about a year now. There is no way to delete a single post without deleting the entire feed. What is probably happening is that you have disabled "Read" articles in the filter:
<details><summary>Alt Text</summary>Screenshot of Feeder main screen. Shows filter menu opened displaying checkboxes "Unread", "Saved", "Recently Read", and "Read". All options are checked.</details>
Gosh you're exactly right! I had "recently read" checked which sounds self explanatory but the two work slightly differently. This gets me one step closer at least.
Unfortunately, the Pro version is subscription only ($90/year).
I use the free version so I'm limited to 150 feeds, which is enough for my use. And although I like some of the other pro features, I can't justify paying the high subscription price.
Using the pro version for years - they regularly have sales where you get 50% more time per € so it's not nearly as expensive if you stock up for 2+years if they do that