diacetyl in mead?

Unless I have been misled, diacetyl and DMS are unrelated. DMS is created by heat from SMM precursors in malt. Boiling will drive it off, and a 60-90 minute open rolling boil should be enough for most grists. Diacetyl is a natural by-product of yeast fermentation as it is one of the possible intermediate products on the way from sugar to ethanol. Yeast will usually clean it up after the sugar source is depleted.

Causes for excess diacetyl in beer include moving the beer off the yeast too soon and poor yeast health. The same could cause it in mead, though I haven't experienced it there myself. I have had acetlyaldehyde in mead for a few days during fermentation, but it also cleared up on its own.

DMS tastes like cooked corn, diacetyl like movie theater butter.

Diacetyl also get produced by lactic acid bacteria (malolactic fermentation). In winemaking this can be very desirable such as in those nice buttery, barrel-aged chardonnay wines. They produce even more if sugar is available (but also produce VA in that case).

So it is possible that if you used fruit, you had some MLF occur causing more diacetyl.