I had a similar issue with not getting a prism for Crocadille (sp), finally figured out that if you've unlocked a transfig for a Mirage regardless of whether or not you've actually triggered it the game considers you as "having" it in terms of not giving you the free initial prism. So in my case I had I think it was a Cockatrice with the Crocadille transfig option unlocked on the board via level but I had never actually used it, once I figured that out I transfiged to the Crocadille once and it registered as me having it on the map screen where it shows what Mirages you have caught. A bit awkward but makes sense.
In your case do you already have a regular Mu that has the level 10 transfig for Reaver Mu unlocked? If so that's what's happening. I think this is at least in part why on most/all mirage boards where the Mirage can transfig there's a spot on the board where you get another prism for the same type of Mirage, so that way you can transfig in a different direction if say you missed out on the "first" form of a mirage and only got the second version, getting another prism of that second one would let you have a separate mirage you could change back into the first version. If that even makes sense lol.
The game is a little awkward in this regard in terms of how it handles transfigs versus unlocking a new mirage board - like in my example I started with a Cockatrice which can transfig into Valefor, OR you can unlock a completely new board with Cocadille (who I think can then also transfig into something seperate). But the game still considers all of them the "same" mirage in terms of your sync level even though you can't view all of their mirage boards at the same time. When you're at the Prism Case and you can see all of the possible transfig/unlock options right there it makes more sense, but it's another layer of menuing and is just kind of clunky.