Mummy Lords and CR

[Trask]

My party found some magic beans some sessions ago and planted them now. One of the results they got on the table was the mummy pyramid which has a mummy lord inside. My players are level 6 but they decided to go in, allured by the mountains of gold I described within. They did battle with the Mummy Lord, the beginning was bad for them with the rogue/warlock getting reduced to 1 in the first round by Harm. They put up a good fight but their lack of ranged magic and the Mummy's very annoying ability to just whirlwind away all the time really put a strain on their ability to do any damage. But it was very close, and by the end the druid player almost beat it with it coming down to a made or failed save whether the Mummy Lord would go down.

It was a tense, albeit frustrating battle for the players and there was a TPK, but they went in of their own volition so they didnt get too upset. I didnt really have a problem with the way things panned out, if the Mummy Lord died then he died and thats just the way the cookie crumbled. Of course he didnt die but thats just the way it crumbled as well.

My real shock, and the reason I'm making this post, is how completely wack CR is in this game.

Up to this point I've understood its not exactly reliable and I've used a hefty amount of my own judgment. But here was a CR 15 monster that wasnt ambushed, wasnt trapped, wasnt caught at low hp. Full health, full spells CR 15 Mummy Lord and it was a hairs breadth away from defeat at the hands of four level 6 characters. That is just insane, just how unbalanced is the CR is in this game? If four level 6 characters can put the hurt on a CR 15 mummy lord to the absolute razors edge of killing him, how is it a good measure of anything? Is this just a freak incident where the monster is severely underpowered or is it really that messed up? A level 15 party would MOP. THE. FLOOR. with this thing.

What gives?

EDIT: Party was all level 6, dwarf moon druid, elf shadow monk, elf ranger, kenku rogue/warlock