Hmm, I think it's happening again. I changed the Planet Ubuntu to only feed on posts tagged 'ubuntu' from now on, and now as an example tagged one post as 'ubuntu', in addition to this one I'm now writing. Google told me that using "Label actions" to change labels wouldn't affect the feed, but it looks like the feed timestamp for my previous, older post was again updated (not the "Published", but "Updated")... so I guess I won't be tagging any further old posts, unless it turns out the previous problem I had was that the Published stamp was also renewed and the Planet Ubuntu ignores Updated stamp.
If you have a solution, please tell it.
Update: Solution found! Thanks, artfwo.