Hmm I actually didn't notice the interest for HBD has dropped to 15% until Hivians start to write about it recently. Looking at my wallet and the pending interest, I did a rough calculation and it seems to me that the 15% has being going on for at least a week or two. I remembered it went down to 19% earlier this year as well and then it went back to 20%. For me, am not making changes. It's still 50/50 split between Hive/HBD for my posts.
Its just been for a few days now, but the way the interest is calculated, if you claim interest now, it will be at 15%, even if for 25 days it was at 20%.
I see, interesting, didn't know that. Thanks for sharing!