Title is a bit dramatic but I think there are opportunities for tuning here.

Player situation: You play a dungeon 2+ levels outside of targeted dungeon level and notice significant impact to difficulty.

I believe this is due to a mechanic against higher level mobs that causes you to miss more and crit less and against lower level mobs that allows more hits and higher crit percentage. The damage received is the same, at lower level you get crit on more often and at higher level less. This is all pretty standard mechanic however i think the ramps are a bit too intense.

My experience as a healer: Group has gained no notable skills or gear changes while leveling 4-5 times. Experience in the dungeon at the low end is arduous (and fun, i like the challenge) as damage is very spiky, damage is slow and creativity is required. 4-5 levels higher you go back to the same dungeon content and I literally dont have to heal on most encounters and dps as it is faceroll easy.

Situations where i experienced this:
T1 dungeon group at lvl 10-11 - hard content
T1 dungeon group at lvl 13 - content moderately easy
T1 dungeon group at lvl 15+ - i can solo many encounters
T2 dungeon group at lvl 16 - close attention to boss mechanics and you can prevail
T2 dungeon group at lvl 18 - content requires intermittent attention
T2 dungeon group at lvl 20+ - you can ignore mechanics and danger and smash buttons

Maybe its more impactful at low levels and as we get higher it will be more flexible but it really seems the dungeons are only interesting as a "check-in" before moving onto new zones/content. Re-playability is low due to reduced difficulty.