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>
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