OK, I threw together a deadbug (see attached), and... same results. Argh! As before, the alternate SPIFlash library works, but the LPL SPIFlash library results in "Init FAIL!". I even deleted the LPL SPIFlash library and downloaded a fresh new one from LPL github repository. Same results.
In case you're wondering, the Winbond flash chips came from Digikey, so presumably no problem there. For the deadbug I used a different Winbond flash chip, just in case the previous two might have been dodgy. Obviously, that made no difference.
If it *is* a capacitance issue, then maybe the flash chip's breakout board is the source? I'm not sure I have the skill to deadbug without it, but I suppose I could try. But if that's the case, why on three different hardware testbeds now does the diagnostic SPIFlash library work and the LPL SPIFlash library fail? I don't get it: if it were a hardware problem wouldn't the diagnostic SPIFlash library fail too?