When you choose a 2024 species and a 2024 background, the ability score increases from the species are hidden. If you choose an old or custom background, they become visible.

When choosing a homebrew species and a 2024 background, the ability score increases from the species are not hidden. I have the species's ability score increases set up exactly how the 2024 species have theirs set up. I know I can just tell my players to pick one, but I'd like it to work how the official 2024 species work.

Modify homebrew to remove the ASI. 

If they select  2024 background, they choose via the background. If they choose a 2014 background the Abilities tab will let them choose a Tasha's style ASI (+2/+1 or +1/+1/+1). 

I just tried that and it doesn't work.

Then you did it wrong, because yes it does.

Did you mean remove the ASI from the species traits?

Yes.

That's what I did.

Then there's something else you are doing that you haven't described or looking in wrong place.

If neither race or background grants ASI the Abilities tab will have one.

If race doesn't grant ASI but background does the Background tab will have the ASI. You can also choose the ASI from Abilities tab as well.

If race grants ASI but background doesn't then the race tab will have the ASI.  You can also choose the ASI from Abilities tab as well.

A 2014 race with ASI and background with ASI may result in the option to select an ASI for both. You may ignore one or make a homebrew copy of race to remove the ASI.

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This is how it works. I have tested every official race and background across 2014 and 2024 rulesets. It has worked as above in every single test with no exceptions ever presented.

Here's what I did: I copied the official 2024 halfling. In the traits, there is an ASI trait. It appears in the character builder and provides the ASI in the abilities tab regardless of whether I choose a 2014 or 2024 background. I delete the ASI trait, there is no ASI in the abilities tab.

I think you are right about how official species work, but wrong about homebrew.

I am gonna have to apologise here. I am unsure how I missed that in testing. You are right. When you remove the ASI from the 2024 race then the failsafe ASI won't show under Abilities, but if you leave it, then you could end up showing two ASI selections when you pick a 2024 background.

Best way around it is recreate the race from scratch rather than as a copy. That should work fine.

I created a test species from scratch and chose a 2014 background and no failsafe ASI showed up under Abilities.

They have definitely changed something. I have done this test over a dozen times before and the failsafe appeared every time. Now it doesn't, even for one I've created and selected ASIs before.