Perhaps I’m missing something but when I use the post filter, the feed output comes up as a continual paragraph, completely ignoring any HTML coding I have added to the post – including any manually inserted media. Yet, when navigating to the single post page, the coding is intact.
Remember that WordPress automatic Excerpts only take the first x words of the post ignoring any HTML.
If that is your issue, the way around it is to use manual excerpts, where you paste whatever HTML you wish to see in the excerpt. If the manual excerpt box is not present in the post editing page, you can activate the option from the Screen option tab at the top of the page.
If not an excerpt issue, please provide a link to the page showing the problem after clarifying the question at the top