While it could happen, I've never known of an obstruction to occur on the pressure side, downstream of the filter. A pipe collapse could do it, but typically that would also involve a leak.
You're going at it correctly, however. Normal flow on backwash rules out a suction side problem. Abnormal (unchanged) flow on recirculate rules out problems with the filter.
I'd look at whatever's aboveground and downstream of the filter.
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