I found this thread that seemed to suggest that by adding multiple, separate prerequisites to a homebrew feat, it should work as an ANY/OR rule i.e. if one of these prerequisites are met, the feat can be used. This pinned thread also says that adding separate, multiple prerequisites should allow this to work:

Meeting one of several requirements: If your feat has multiple requirements, but a character only needs to meet one of them (for example, their Intelligence must be at least 13 or higher OR they must have the ability to cast at least one spell), you would add a new prerequisite from the Feats overview itself.

I'm trying to create a feat that is only available for specific Sorcerer subclasses (Draconic Bloodline, Divine Soul, Shadow Magic, Storm Sorcery, Wild Magic). When I add these as separate class prerequisites in the feat and try to add this feat to my Wild Magic Sorcerer, it shows up as unavailable - "Missing: Draconic Bloodline or Divine Soul or Shadow Magic or Storm Sorcery or Wild Magic".

Am I doing something wrong, or is what I want to achieve not possible?

EDIT: A caveat is that I don't own any of the DndBeyond sourcebooks, but I have access to the Wild Magic sublclass from content sharing in a campaign. Could this be why I can't select the feat, even though my character has the Wild Magic subclass selected via content sharing?

Subclasses are not functional as prerequisites for feats.

Ohhhh really? So even though they can be set as a class prerequisite in the UI, they don't actually do anything? What exactly happens when one is set as one? Does it simply just cause the prerequisite to never be met?

It simply doesn’t work as you’ve noticed.

Just saw this thread that you also commented on. I'll just add my bug report there for the sake of it.