The reporting on this case has not been easy to decipher best I can tell, his ex-fiancée is using language that would indicate that at least she is attempting to use sovereign citizen tactics. Here is a link to the affidavit she filed. Here are a few screenshots:
According to a reddit post about the topic, both and his ex-fiancé have court filings against “against the county’s child protection service agency with a barely coherent complaint to the USDOJ / “Pleas of the Crown” and “Acts of Treason” claiming he and the children’s mother are “foreign nationals but not citizens of the United States at birth.”
I have not seen these records to verify whether the above is true so at this time, we do not know if Brown himself has tried these tactics or if it is only his ex.
Will post if more information becomes available.