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Are you getting PP5V_S5?
L7520 seems to be powered by PP5V_S4RS3. If PP5V_S5 is missing then I dont think you would get P5VS4RS3 Have you checked the P5VS4RS3_EN on U7501?
I've only had a quick glance so I may be reading it completely wrong.
History on the board? Is it power cycling, check carefully on L7520 best done with a scope as multimeter are too slow to catch. Inspect usual areas U6100, U1950 pin 8.