You seem to be pointing at the "indication" side of the mystery in yet you said that you tried both electric and mechanical gauges which "indicates" to me that you got something in the passage that feeds the fitting to the
oil pressure pickup as you stated that the motor had full pressure at all times.
Silicone? An obstruction could allow the pressure to initially show high and then drop...
This is one of the things I like about a forum like this. You can learn from other people experiences.
Regards,