Cannot invite a tester to my testflight build, 409 error

I'm trying to add a member of my team to the Testlfight app because they're saying that they were removed. If I go to the Testflight group and try to add them there, they don't show up in the list despite being a tester. If I try to add them as a tester by finding them in "All Testers", selecting them, and doing "Add to group", I get the error:

"An error has occurred. Try again later."

The API request and response looks like this:

Request URL: https://appstoreconnect.apple.com/iris/v1/betaGroups/[redacted uuid]/relationships/betaTesters
Request Method: POST
Status Code: 409 Conflict
Response:
{
  "errors" : [ {
    "id" : "7d7f28bc-60e5-438e-9a29-f17e14b533bc",
    "status" : "409",
    "code" : "STATE_ERROR",
    "title" : "The request cannot be fulfilled because of the state of another resource.",
    "detail" : "Tester(s) cannot be assigned"
  } ]
}

I don't think this person was ever removed, so it looks like their account is stuck in a state where they're in the group but also not in the group. Any ideas for a fix?