I have related issue - cannot obtail "Dragon Scale" from Mystical Agradditions.
I kill dragons several times, have >20 hearts, but non of these Scales.
Dragon drops some "Dragon Scale" from Tinkers Construct mod, that may be the source of problem. But these scales are not compatible in crafting recipes.
As a solution - maybe add them to spawn shop?
BTW about not dropping Dragon Hearts - have same problem, periodically. I guess server global entity limit (or similar option) cought affect this. Because when dragon not dropping hearts, it not dropping any other loot too. Also, vanilla spawners not spawning any mobs. After some time or after restart - everything returns to normal.