If you had a vapor leak prior to the installation that just dissipated into the attic air and was vented out, it could possibly condense on the material you installed.
Take a look next winter and if you see a spot, fix the vapor leak with a can of sprayfoam.
Usually attic mold is an issue if you have; a bath fan blowing into the attic, a basement with a big water issue, a totally unvented roof, some odd moisture load in a house like a pot farm or a combination of things that add up to something.
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