I don’t know if plain bearing oil drainage is a function of RPM, but it may be.
I think it must be.
The oil arrives at a hole in the bearing and from there it flows around and escapes sideways. So, on average, it will initially flow (out of the hole) on a 45 degree vector. The eventual exit from the bearing will be sideways, obviously, so on a 90 degree vector. And the velocity along this vector will be the vector sum of the velocity of the oil flow (which is perhaps constant, since the oil pressure is regulated by the spring loaded valve upstream) and the velocity of the rotating bearing parts (which is rpm-proportional).
That’s my intuition, anyway 
Oil is a fluid. Continuity applies. The only thing that drives the oil out of the bearing is the pressure difference between the oil inlet hole in the bearing and the other edges of the bearing where the oil finally “slides” out. The RPM doesn’t change this, but there may be some secondary effects. How these secondary effects work, I don’t know, but I cannot imagine they are large enough to change the inlet pressure to the bearing. A bearing is no pump in any sort of way.
The tangential velocity direction is 90 degrees to the exit direction of the oil.
Foam, cavitation, partially blocked inlet to the pump, prop governor, or simply a pressure release valve with an “odd” behaviour (by design or otherwise) would be my bets.
OK; let’s look at it another way.
Take the extreme case of a stationary shaft, which is sitting right on the oil feed hole. The oil will try to lift the shaft off the bottom and will probably exit mostly near the bottom of the bearing. Now take the case of a rotating shaft, which sweeps the oil around the bearing and now the oil will be exiting all around the bearing. I reckon this will offer less resistance to the incoming flow, thereby increasing the oil consumption of the bearing.
The oil is exiting all around the bearing already from 2-3 rpm (given it has oil pressure at that speed). There are no forces except the oil pressure that pushes it out. The rotation is in tangential direction, but it exists in z direction. A bearing is no pump.
Two factors that are affected by RPM are centrifugal force acting at the big end bearings and oil heating/viscosity change. I’m not convinced that either of those would result in greater oil drainage flow but having designed a number of machines with plain bearings, and had unexpected trouble with them, I’m aware that bearing design is more complex than one might expect.
I wonder if this has something to do with the prop governor.
The lower the RPM, the higher the pressure in the hub that pushes the prop from the fully fine stops into the position required.
Does the prop governor “leak” oil i.e. is there any steady flow into it?
I had the impression this is very usual for CSU, especially, when recycling prop RPM using the blue stick, I always notice oil pressure changes (while checking no oil splashes on the windscreen from the prop, wonder if those leaks of hot oil work as prop TKS :) )
Also spotted the same behaviour on DA40 FADEC engine when changing RPM via power stick
In theory, the valve that controls the pressure in the governor is closed completely at the target speed, and is open on the pressure side if too fast, and relieving pressure to the sunp if too slow.
But of course there will be tiny RPM variations the valve will need to correct, leading to a bit of oil flowing through the governor.
I have no idea if this is significant at all. probably not, since the oil pressure is much higher than the pressure in the hub.
