FWIW, in 2012, I had this issue in 2012. I had the DL Plat. I spent $25k, got the boost. Got up to $50k (still on Plat) and got second boost. Then, I switched to the Reserve card (somewhere around $60-65k in spend), crossed the $80k threshold in Dec. and got a boost.
So, in my experience, the spend does NOT reset. Anything not applied to a boost counts toward a boost at whatever threshold applies with your current card.