You pinged me on it while I was in the middle of a trip, and when I went to dig into the issue, the rabbit hole went a lot deeper than I could devote at the time.
Excuses aside, it does appear that the primary reason for the current behavior is to match existing prior behavior.
That said, I'm not sure there is a good way to offer an upgrade path to change the behavior, as it is the sort of thing that would just silently break very large chunks of code in very subtle ways with the user having no reason to suspect that a change in the underlying semantics of the mtl was to blame, so I'm rather reticent about just diving in and "fixing" it as basically every existing user of the instance is pretty much guaranteed to have breakage.
-Edward