I did notice the obverse head type was just like the one above this coin (in Wildwinds). Could it be a "mule"
No, not a mule. Mules refer to obverse and reverse types that don't belong together, such as on a coin that has the obverse from one emperor and a reverse produced from a die created for another, perhaps earlier emperor's coinage.
Since both coins you refer to are Mantineia emissions for Plautilla, it would be more correct to refer to the identical obverses as an obverse die linkage, assuming the dies are indeed identical. If the dies are similar, but not identical, then it may well be that both were produced by the same die engraver, and those both exhibit his distinctive interpretation of Plautilla's likeness.