I've seen dozens of coastal and interior AK black bears hit the dirt and I've never seen a legit 20+" skull in any of them. Some were close while green, but they didn't make it after drying.
My AK taxidermist has seen and handled hundreds if not thousands over the last decade and can count on one hand the amount of black bears that broke 20" and I believe one of those was right near 21". That said, his sample size is limited to mostly interior bears. Not as many from Southeast.
There could very well be pockets in the eastern states that regularly produce 21" record heads, but they are hard to come by in AK. IMO 20" heads need to be "All Time" records.