Add profiles to list failed with Invalid profile IDs: set()

I’ve been running into an issue trying to add profiles to a list in batches of 950.  Things were working fine but I started to run into this error:

{:success=>false, :response=>{:errors=>[{:id=>"c8932474-507f-45e6-9d12-4a1a87403ae1", :status=>400, :code=>"invalid", :title=>"Invalid input.", :detail=>"Invalid profile IDs: set()", :source=>{:pointer=>"/data/"}}]}}

The endpoint is:
https://a.klaviyo.com/api/lists/{id}/relationships/profiles/

And I tried to keep my payload under 1000 so 950 per call.  I’ve been able to successfully move 20,000 profiles into a list without issue but lately I’ve been getting the error above.  Any ideas what might be happening?  I was going to drop my numbers down to try to isolate which profiles might be failing.  Would love any feedback if anyone has hit the same error.