Mondonico was (and maybe still is?) a top-drawer builder. There are plenty of Mondonicos out there that are badged as such. If it was Masi-designed and Mondonico-built, I don't think that makes it not-a-Masi. (Same for a Billato-built Masi-designed and badged bike.) A number of Italian (and other) frame makers contracted their designs out to other builders to build. In a few cases, the result was inferior build quality. In most cases, the build quality is equal to that of the builder whose name is on the frame. Occasionally, the build quality was better. Unless there is a known issue with a contract builder turning out an inferior product, I think worrying about whether it is a "real" Masi or whatever is a rabbit hole best avoided.

Full disclosure: I buy (well, bought) my frames to ride, not as collector pieces. I was/am kind of particular about what I want (mostly what I lusted after as a teen and couldn't afford then), but I like to play with my toys as well as look at them. Clearly, a collector will value a frame with proven provenance that a Whatzit frame came out of the Whatzit factory - and better yet, was built by Giuseppe Whatzit himself - over one that was contracted out to Biamazetta or Billato or any of the other good contract builders. For a rider, I seriously doubt it makes a difference in the vast majority of cases.