It doesn't work like that. Timezones are taken into account in date stamps. So any article will become immediately visible regardless of timezone differences as long as the timestamp of the article is not in the future. For example if some feed contains an article timestamped to year 2124, it won't become visible before that year is actually active. It sounds like in this case the feed posts some articles so that they are datestamped to the future.

If you want more clarity with this, please send the feed URL and your timezone info to feedbro.reader@gmail.com

EDIT: the reason Feedbro doesn't load articles that are timestamped to the future is that it would complicate many things. For example if you have a Rule that would popup a notification when a new article arrives, should that Rule trigger when an article is loaded that is timestamped in to the future? Probably not but then when should it trigger? That would then require some background scanning process to find articles that haven't been triggered by rules but are already in the database... etc. It would complicate and slow down the code quite a bit. Unless the feed is super floody, Feedbro will eventually load the article so it shouldn't really be an issue. And this is relevant for maybe 0.001% of the feeds out there so it's an obvious WON'T FIX decision.

The feed probably has markup that has hardcoded font size?

Feedbro does not cause the corruption. IndexedDB corruption can occur when Firefox updates. The profile/IndexedDB functionality completely breaks (has nothing to do with Feedbro) since it breaks for all add-ons and websites. There are bugs about this in Bugzilla.