So I don't know if this is a bug or not, because I'm not entirely sure if this counts as a monk weapon.

Monk says:

At 1st level, your practice of martial arts gives you mastery of combat styles that use unarmed strikes and monk weapons, which are shortswords and any simple melee weapons that don’t have the two-handed or heavy property.

Fanged bite says:

Your fanged bite is a natural weapon, which counts as a simple melee weapon with which you are proficient

So, a monk weapon is any simple melee weapon that isn't heavy or have the two-handed property. According to fanged bite it totally fits. It's a simple melee weapon and doesn't have any properties that would disqualify it.

It does however use Constitution for the attack, which in my opinion would mean that I can't use the monk ability that lets me use Dex, because that specifically says you can use Dex instead of Strength, and since Fanged Bite uses Con and not Strength, it doesn't qualify to use Dex instead. Which is fine. 

So in my opinion the fanged bite should count as a monk weapon? But in D&D beyond it's not scaling with my monk's martial arts die.  Is this a bug, or does the fanged bite not count?

Thanks!

I found this post in the Ravenloft sourcebook support thread. It looks like you're correct that it counts as a monk weapon, uses CON, and also that there is an issue with it not using the martial arts die, but there apparently is a workaround.