(Tested with different users, not unique to me)

Upon character creation, all Nature Domain clerics are incorrectly and automatically getting two Homebrew cantrips: Bitter Blast and Winter's Return. These cantrips cannot be removed (attempting to remove them yields an error message).

Example of character with them (this character has Homebrew toggled to "Off" in the character manager btw): https://www.dndbeyond.com/characters/120141733

These spells don't exist in any official sourcebook or adventure that I can find on DNDBeyond, but I did find them by searching the Homebrew section. Both were created on 7/24/20 by a user named ImpLaughs:

Bitter Blast: https://www.dndbeyond.com/spells/584418-bitter-blast

Winter's Return: https://www.dndbeyond.com/spells/584406-winters-return 

That’s because someone in your campaign has that cantrip in their homebrew collection, and whoever wrote it added that subclass to the cantrip under the list of classes. Please see the Homebrew Spells FAQ #1 for additional information: (https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/131411-a-homebrewers-how-to-faq).

Quote from IamSposta >>

That’s because someone in your campaign has that cantrip in their homebrew collection, and whoever wrote it added that subclass to the cantrip under the list of classes. Please see the Homebrew Spells FAQ #1 for additional information: (https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/131411-a-homebrewers-how-to-faq).

Thanks - I will investigate to see if that's indeed the case here.

Even so, shouldn't toggling Homebrew to "off" in the character manager logically stop another player's Homebrew spells from inextricably populating the character sheet?

One would think so,, but no.

Yup, confirmed that this is indeed what was happening. Thanks, @IamSposta!

I'm still gonna call that a bug. Toggling Homebrew to Off should mean Homebrew is Off. 

Happy to help. It happens because whatever subclasses are listed as attached directly to a spell like that are meant to be “subclass granted,” “always prepared” spells that those characters simply always have access to. In the course of programming that function, they inadvertently left a back door open that lets homebrew spells sneak through like that regardless of homebrew settings for the sheet. 🤷‍♂️