The thing is, when it comes to TakaraTomy-original Transformers media, when Takara wants us to know a character is female, they make out loud and clear in an obvious way, since this is still considered a "Boys' Toys" property first and foremost over in Japan. Female characters don't get equal treatment to male characters over there, sadly. If a character is female, there is no ambiguity. The show would make that clear, and any bio or profile would likewise use unambiguous female pronouns to indicate female characters, whereas male characters tend to get gender-neutral pronouns in their Japanese bios and profiles. That's just how Japan tends to treat girl characters in Japanese-original properties aimed primarily at young boys.
Take the overly stereotypically girly portrayal of Cybertron Thunderblast as a glaring example. Or how Arcee was portrayed in Q-Transformers as overly cutesy and blushing in embarrassment and shyness just because she was a girl (the
only girl in the show, even). Or, hey, even how Transformers: Go! had z
ero female characters in it at all.
Back up to this particular case,
the bio for Thunderbeak on Takara's website uses no female pronouns or gives any indication of Thunderbeak's gender at all. If Thunderbeak was female, the bio would absolutely go out of its way to point that out and say so, unlike bios for male characters where "male" is just automatically assumed by default because "Transformers is for boys" in Japan.
Hasbro has come a long way in giving us more and more female characters, but Takara on their own is still lacking in that department.