- Going by the old anime, he is at first forcibly transformed into Tuxedo Mask when Sailor Moon is in danger with no memory of what he did as Tuxedo Mask when he changes back...until he starts gathering Rainbow Crystals. This happens to no other Sailor Guardian, not even Sailor Moon who was not even a Guardian in her previous life, and not even to Sailor Saturn who had to deal with Mistress 9 as well. After the first arc, Mamoru loses his memory, and his subconscious will to protect Sailor Moon manifests as the Moonlight Knight...something that happens to no other Sailor Guardian. There are only a few times a voluntary transformation is actually shown, and unlike the Sailor Guardians, he doesn't seem to correspond to a specific element or attribute. He throws roses, has a cane, and that's essentially it.
As a rebuttal to this, what about Sailor V? None of the other Sailor Guardians has a "V" form... If Minako can have this weird, unexplained version of her Sailor self, why can't Mamoru have his own Sailor weirdness?
At least in terms of the manga, in Infinity we're told Sailor Saturn will only awaken when the three talismans resonate with each other...but then in the Dream arc, she's able to awaken again just because everyone's in peril, so there really isn't an issue with the circumstances surrounding Mamoru's ability to transform into Tuxedo Mask (whether he's physically putting on his costume, magically transforming, or being transformed without his knowledge) changing.
To be fair, he does have psychometric powers (at least in the manga), so he does have a specific attribute, if not a specific element. (But then, Venus doesn't really have a consistent, clear element either: is it love? light? metal? And what about Moon? Is her element love? light? healing? crystals? mystery? rainbows? therapy?)
Basically.... Inconsistency is the only thing that's been consistently consistent about this franchise since its inception, so that there are inconsistencies with Mamoru/Tuxedo Mask as a Sailor Guardian actually shows that his being a Sailor Guardian is entirely consistent!