Blowing a 10-point lead in the "3-pointers or bust" era isn't a big deal, as it takes only 3 shots to pretty much make up that deficit, but blowing a 30-point lead is a good bit harder to do, even with all the threes.
But blown leads are just one part of Boston's larger "end of quarter/end of game" issue—blowing leads, offense stagnating to the low-efficiency (and entirely predictable) Tatum isos, allowing opponents to go on miniruns that significantly cut into Boston's lead.