active forum topics block cache gets very stale?

Hi everyone -- I have extensively searched trying to figure this one out but found nothing. I recently upgraded to 6.3 (from 4.7 -- yes, I am a slacker ;-).

The performance of my site has improved vastly with the new version, which is great because performance had really degraded. I am using block caching among the other caching types (not aggressive).

My issue is that for anonymous users, and sometimes maybe even for logged in users (haven't confirmed this), the "active forum topics" block content gets very stale -- like over a day old on a site in which new comments are being posted several times and hour and new topics several times a day.

Other blocks may be stale too, in theory, but i don't know because the forum topics one is the only one that is very frequently updated -- so i don't know if this is a generic block cache issue or if it's specific to active forum topics.

I have minimum cache lifetime set to 5 minutes or so.

Manually flushing the cache works to fix this.

I have not been able to find any evidence of this happening to other people... have I done something wrong, or am I misunderstanding how block cache is supposed to work for a block like active forum topics? I'm afraid to turn it off because i like my speedy new site performance.

Thanks!
Rich