I think this may have worked itself out. I cancelled my recurring subscription via credit card the other day. When I did, I noticed that I had 32 days of sub time left.

Based on my last sub payment date and any time I have already used, the 15 days of free time looks to be factored into this. The only reason I wasn't seeing it before was probably due to the recurring sub not showing it. This doesn't solve the problem that I never got notification that I earned any free time, but that's a different matter.