Long winded again… I seem like I do that (regrets)
I think it's a combination of many factors adding to increased oil flow resistance in this case.
1) The current sandwich plate (photo) seems to have smaller cooler lines (1/2 inch?)
2) With a typical cooler installation, there is 3 - 4 ft of cooler hose (or more) - longer lines more resistance,
3) No oil thermostat, ALL oil passes through the cooler so for a long time oil is at or only slightly above ambient temp (likely < 100 F) – thicker oil more resistance,
4) Oil pressure release valve is BEFORE the cooler so, with excessive flow resistance some part of the oil dumps back to the pan...
Hint - if you are really concerned about this one due to lots of cold weather use (winter rally car?), use an external pressure release valve AFTER the cooler and you will see engine oil pressure releasing at closer to normal when cold… (See Jegs or Summit for parts) Tee into the cooler return line just before the filter (or equv at sandwich plate).
OR cover the cooler so it has limited air flow.
Stock Mazda approach solves lots of issues here:
With no sandwich plate, cooler or cooler lines AND the stock cooler warming the oil with water as the engine warms up. Items 1 – 4 are all not an issue.
Oil heats faster… Great for keeping the oil warm in an autocross application once the water is warm.
Elsewhere Savington started an Oil Cooler thread. Lots of good stuff here:
Oil cooler tech
I stated in the above thread that mounting the cooler “ports up” will keep the cooler full so there is no concern with it draining back to the pan when parked (especially in a race engine that may go weeks between startups). Full cooler less chance of a low pressure or dry start… As the Oil Filter is AFTER the cooler, the anti drain back valve there is of no benefit… See sketch.
Several configurations in attached images... Mount your cooler as you see fit, I'd mount one ports up.
Also note, the oil thermostat ”Opens” the bypass when cold and “Closes” the bypass when hot.