First signer of multple not processed with FormFieldLocation

We're using the rest API and inserting FormFieldLocation per signer.

When a agreement is created and multiple signers we enclosed with different "Form Field Locations",

the signing block for the first signer is always placed at the bottom of the document.

So Adobe Sign is not using the FormFieldLocation for the first user:

var formField = new FormField
{
Name = $"Signature ({member.Name})",
InputType = FormField.InputTypeEnum.SIGNATURE,
ContentType = FormField.ContentTypeEnum.SIGNATURE,
Required = true,
Assignee = participant.Id,
Locations = new List<FormFieldLocation>
{
new FormFieldLocation
{
PageNumber = 2,
Top = 100,
Left = 100,
Width = 150,
Height = 30
}
}
}

All the others signers are placed correcty in the document (so page 2 instead of the last page 4).

What do I have to do to have the first signer placed correctly?