If the filter was not a genuine Subaru ATF filter that could be the problem. The subaru filter only filters like 10-15% of the fluid during each pass. Engine filters fit but they tflter 100% on every pass and that drops pressures/flow.
Thats not the issue. Even with clean fluid, filter differential pressure is to high.
Unfortunately people never change their ATF and once it goes south, new fluid probably won't fix it. I did a drain and fill every 15 to 2K miles on my 08.
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