Why does the Microsoft Graph API's invite endpoint only return share link URLs for certain email addresses? - sharepoint

Question

When I make requests to the Microsoft Graph API's invite endpoint to create secure (email-based) sharing links for a specific collection of emails that are external to our organization, the response does not contain the requested sharing link for certain email addresses. When I look at the SharePoint site UI to see how the permissions are listed for the item, the email address with issue (RealEmail) shows up differently from the one that does return a sharing link through the API (TestEmail). Both RealEmail and TestEmail are marked as external users, but it seems that the RealEmail external user is recognized as an actual external or guest user account, rather than just being an email unknown to SharePoint, like TestEmail.
Here's the request I made on Microsoft Graph Explorer:
POST https://graph.microsoft.com/v1.0/sites/{site-id}/drives/{drive-id}/items/{item-id}/invite
{
"requireSignIn": true,
"sendInvitation": false,
"roles": [
"read"
],
"recipients": [
{
"email": "RealEmail#example.com"
},
{
"email": "TestEmail#example.com"
}
]
}
And this was the response (with real data removed):
{
"#odata.context": "https://graph.microsoft.com/v1.0/$metadata#Collection(permission)",
"value": [
{
"#odata.type": "#microsoft.graph.permission",
"id": "removed",
"roles": [
"read"
],
"grantedTo": {
"user": {
"email": "RealEmail#example.com",
"id": "removed",
"displayName": "RealFirstName RealLastName"
}
}
},
{
"#odata.type": "#microsoft.graph.permission",
"roles": [
"read"
],
"grantedToIdentities": [
{
"user": {
"email": "TestEmail#example.com"
}
}
],
"invitation": {
"signInRequired": true
},
"link": {
"type": "view",
"webUrl": "removed"
}
}
]
}
This is what the Manage Access UI looks like (with real data removed)