I have never seen a problem where the oil pressure on an engine would drop as engine RPM was increased.  The only explanation that I can think of is a mechanical failure of the oil pump.  As I recall from your previous posts:

1.  Your engine experienced significant dilution of the engine oil with diesel fuel followed by at least two runaways which likely exceeded the maximum RPM rating of the engine.

2.  Apparently due to the loss of lubrication, the oil pump was badly scored.

  My guess is that due to the lack of lubrication,  high engine speed (and perhaps some galling of the oil pump which lead to the  scoring the oil pump) that something in the drive for the oil pump has mostly failed.  Perhaps the gears driving the oil pump are beginning to slip at 2000 RPM now because either the gears or the bearings that should hold the correct clearances have been damaged.  In other words the gears could be only partially engaged due to wear of the gear or the bearings and as the oil pressure rises with engine speed, you reach a point where they begin to slip, causing a rapid drop in oil pressure.  Lowering the engine RPM and oil pressure might be allowing the pump to resume turning as the load on the pump drops and the gears partially reengage.  Certainly unless this problem is corrected, your new engine will not last long.   It is of course impossible to know for sure what the problem is but given that the oil pump is essentially a positive displacement pump which should pump more oil with increasing RPM,  this is the only scenario that I can think of to cause the symptoms that you described.

Best of luck and Happy New Year,

James

SV Sueño,  Maramu #220