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.