Sort:  

This only happens if the haircut rule is triggered, meaning the market cap of STEEM is too low to satisfy the SBD debt level, meaning no more SBD is being printed, which is why you weren't paid out in SBD.

There's a small calculation on https://smartsteem.com/promote/buy, if you click on Show Detailed Payout:

Ahhhhhhh, thanks Wolfie! I've been around for a while but I didn't realise that was a thing. Totally makes sense. Thank you!