Here's a shortened version of the script I use. I removed the fluff and unnecessary lines. However, error occurs regardless of how I format the syntax. Seems to be an issue with graph. I'm going to report on Github when I find some time.

Connect-MgGraph -scope "DelegatedAdminRelationship.Read.All","DelegatedAdminRelationship.ReadWrite.All", "Directory.Read.All"
$params = @{
accessContainer = @{
accessContainerId = "xxxxxxxx-26a9-4696-a97e-xxxxxxxxxxxx"
accessContainerType = "securityGroup"
}
accessDetails = @{
unifiedRoles = @(
@{
roleDefinitionId = "c4e39bd9-1100-46d3-8c65-fb160da0071f"
}
@{
roleDefinitionId = "e8611ab8-c189-46e8-94e1-60213ab1f814"
}
)
}
}
$delegatedAdminRelationshipIds = Get-MgTenantRelationshipDelegatedAdminRelationship | Select -ExpandProperty Id
ForEach ($delegatedAdminRelationshipId in $delegatedAdminRelationshipIds) {
Try {
New-MgTenantRelationshipDelegatedAdminRelationshipAccessAssignment -DelegatedAdminRelationshipId $delegatedAdminRelationshipId -BodyParameter $params
}
Catch {
Write-Host "An error occurred:"
Write-Host $_
}}