I applaud your results with autofocus using the 90mm with or without add-ons
As I have found AF to be very unreliable with this lens for the very close subjects it was designed for. Even with the OM-1 and latest firmwares, settings checked and explored, etc
It could be that the lens is getting distracted by microplankton, or that it just loses things easily near the tight 1x to 2x magnification range. It happens on land too, so I doubt it is anything an in-water MFO is supposed to improve.
A failed AF focus hunt happens often enough that decoupling the AF from the shutter button ended up being a requirement with this lens. Do I have "a lemon?" I don't think so, as the lens is otherwise very good and does perfectly well at non-supermacro ranges.
I operate this lens in manual focus mode with peaking enabled. The AF trigger is mapped to a separate button only for when I am lazy, rushed, or "feeling lucky." (The gamble often loses)