I thought this was changed after months of complaining to be the case.
Why then am I fighting on Mayhem 10 and getting straight level 57 items?
So sad for Gearbox this lesson wasn’t learned from past experience…
Treating mayhem levels separately in this regard is BS.
The item scaling (or lack there of) will be the bullet that kills this game … for me anyways.
Some bosses have issues, mainly due to the drop pool or part of being classed as a chest drop which is not yet levelled but planned to change in a patch.
Wotan has been fixed in todays hotfix but depends what you are farming.
I’ve had this happen from every boss that I’ve killed since Mayhem 2.0. I don’t think that it’s a specific boss issue.
It’s possible they all work the same way but it’s more noticeable as it happens on certain bosses like the Warden. You can see two distinct drop phases on a kill and iirc the first one is always the underlevelled item. If it all magically gets fixed come phase two it will be a lightbulb moment. 
My point is, since they had the issue before, prior to Wotan even being released.
Then addressed it to be max level character receives max level item drops.
This should never have been happening at all…
If it is a simple mistake or oversight, it is incompetence.
If they don’t consider Mayhem levels to be actual levels, then why bother.
You can look at the design element reason why, but why does that matter.
To correct an issue at the community request, then revert back to the same model makes a fun game so frustrating people will leave… oh wait…
This is to me one of the biggest disappointments about Mayhem 2.0.
In multiple ways it regressed the game. After months of work on (among other things) Iron Bear and loot balancing, they release 2.0…which utterly breaks Iron Bear, unbalances the loot, and reverses the only-just-provided max level play = max level gear guarantee that the community has been begging for since the release of Borderlands 2. <— Not a typo.
So much about 2.0 was and remains best summarized as follows:
