All of this is due to very poor adjustments of the HP of bosses/mobs between levels 50 and 80.
For the level 1-50 instances, it's obviously because of the current class balancing synced to low levels with spells that are way too powerful.

They absolutely need to rework the synchronization of the old instances to make them more similar to the original experience.
It might be impossible to fully replicate the original experience, but at least they should adjust them so that players can experience all the mechanics and not skip 80% of the boss mechanics.
It's a huge waste actually.