Dragonborn and gentilas - don't seek reason where there is none. And never has been.
Look no further than oviparous Githyanki who have b00bies (females) and belly buttons (both sexes).
I mean technically... githyanki evolved(!) into oviparous. B00bies and belly buttons could well be explained as vestigialities.
Belly buttons don't make sense even as vestigial. Unless each Gith also grows a random external placenta that needs to be severed upon hatching (This placenta would be doing literally nothing. Since there is no mother feeding nutrients to the feotus through it)...
Honestly, belly buttons on most fantasy races doesn't make a lot of sense, given the unique nature of human feotal development (Not all mammals have the same issue of a vestigial placenta that needs to be severed from newborns which creates the scar known as a belly button)
But they're often there, because the trope of other races just being "Humans, but..." thus being simply modifications of the default human form rather than actually unique races.
Where are sources that can prove that BG3 is not canon, or at least has elements that aren’t considered canon
BG3 as an officially licenced campaign is canon.
However, the canon for DnD is irrelevant outside other officially licenced campaigns. As for home use, any materials are simply guides for players to utilize. Ultimately whatever the DM writes or utilizes, is what is canon to that specific campaign. Whether this is using source material to the letter, or heavy amounts of homebrew.
Thus, unless you're literally writing an official campaign on behalf of WotC, anything you want is plausible.