Hi Brian,
http://www.seacoastbeveragelab.com/feed/podcast/
and
http://www.seacoastbeveragelab.com/?feed=podcast
are basically the same. The first URL is only a variation of the second URL. Both lead to the same content.
I do not know why the validator showed you an incomplete feed. But if you try that again today, it will show you the last post too.
One reason this glitch maybe your Cache plugin which may have delayed the delivery of the last post via the feed. But I'm not sure of that.
Tim