Could be some weird under the hood thing, but could see needing to retain the different Tatsugiris in case it does interact with Dondozo and they want to retain the Order Up stuff.Could be, but then I would wonder why the data makes it look like Tatsugiri and Magearna have separate Megas for each of their forms whereas Meowstic does not (unlike Pyroar, Meowstic’s gender differences are actually treated as alternate forms).
If it were just a matter of compatibility like with how Mothim technically has Sandy and Trash Cloak forms in the code (that just look identical to Mothim) so that each Tatsugiri has an identical but separate model to Mega into, I would think it would need to be applied to all Pokémon with multiple forms that can Mega Evolve.
Personally, I think on some level, GF see regional forms as effectively being new species, so I could believe that Tatsugiri’s forms could each get their own Mega while Galarian Slowbro doesn’t get one at all.
Magearna is the stranger one, though. Original Color is just a color swap. If you can do a single form for the Meowstics, does feel like you could do it here. Unless, again, there's some quirk under the hood how even though the Male & Female are different forms they're still "treated" as just a gender swap?
& again the oddity with the form listing not scrubbing the listing like it did in the zukan comments.