I discovered today that I have access to far more races under Game Rules>Races than I should.

I am not in a campaign with content sharing turned on.  The only race-related purchases I have made here are the PHB and the Tabaxi.

But if I go to Game Rules>Races, and click on the “View X Details” where X is a race, I am taken to the detail/compendium-like pages for all the Races in Volo’s (remember, I only “own” Tabaxi from there).  The same thing is true for the Tortle from the Tortle package (which I also haven’t purchased).  For all the other sources with races that I don’t have (Ravnica, SCAG, Mordenkainen’s) I am correctly taken to the purchase page.  I’m not including Wayfinder’s in this since it’s still playtest.

I do not have any of the races I shouldn’t in the builder; they just show up under Game Rules>Races, then clicking on the appropriate “details” button.

It sounds like you are in fact in a content sharing campaign. You might want to double check.

I did double check before I posted:  I’m only in one campaign I didn’t start myself.  That one has been inactive for a year or more; on the “my campaigns” screen it show up, but there is no “Content shared by” notation on it.  Furthermore, it seems to have been completely partially deactivated: there’s no “view” button on the entry for it on my campaign page.  I just tripled checked, and I can get to the campaign page through the entry for my character in it, but if I try to create an unassigned character in that campaign, I don’t have access to the races I can see but shouldn’t in the Game Rules>Races section.

Oh, and apologies if my reply sounded snarky or exasperated.  That wasn't my intent.  I know why you suggested I double check.  One other time I had access to stuff I thought I shouldn't, and it was because of campaign sharing.  Back when the campaign I mentioned above was active, we had campaign sharing on.  Then the DM discontinued the campaign and eventually turned off sharing (or let their sub lapse, maybe).  After a while, one of the other players who had a Master Tier sub and an extra slot turned on sharing, and I didn't realize they had initially. Eventually they turned sharing off.    That's why I double checked before posting:  To make sure that hadn't happened again.  but you didn't have any way to know all that backstory.

I passed this on to support staff, but it might be a good idea to submit a ticket about this as well.  Here's the support ticket link: https://dndbeyond.zendesk.com/hc/en-us/requests/new?ticket_form_id=602788

Thanks

It looks like a fix was released for this issue earlier today!  Let me know if you're still seeing oodles of races. :)