The reasoning behind this seems to be some hidden scaling aura at work for level 50s, increasing the damage for all spells cast. This aura doesn’t seem to be present for level 60s, making them deal significantly less damage and take significantly more damage from enemies.
To show this effect, we have two logs of two naked Shadow Priests casting Mind Blast against the same boss, Mythic Garothi Worldbreaker.
You can see on both logs that Mind Blast deals about the same damage at 50 and 60. However, the level 50 priest has 156 intellect, while the level 60 has 451 intellect. This means that the level 50 is dealing 3x more damage than they should. Dealing the same damage wouldn’t be an issue normally, but when you take into account that the level 50 can get much more power from Battle for Azeroth’s systems like Azerite Power and Essences, than a level 60 character will be able to at the start of Shadowlands, things scale out of control.
Below are two examples, one of a level 50 and one of a level 60 Shadow Priest against Mythic Garothi Worldbreaker. The level 50 using their Patch 8.3 loadout at item level 130 with Azerite Powers and Essences, and the level 60 character using item level 190 Honor gear.