OM-1 S-AF problem or is it me?

I have been laid up in a chair looking out at my garden for the past week and have been doing a good deal of testing on OM-1 focus.

I have previously reported that S-AF has excellent discrimination when trying to select a bird in a bush, instead of the branches.  That is still true.  It is almost 100% accurate in picking the bird using single AF area.

However now I have found a problem.

My chair is 4 metres from some plants, some of which have tall leafy twigs.  Behind the plants is a gap of 150 meters (the land drops away) to a bank of trees on the opposite side of an inlet.

If I focus using S-AF on the distant trees and then aim the camera at the plants, the plants are so out of focus that there is only a vague blur before focusing.  Pressing half shutter does nothing, no focus change but focus confirmation square in the centre of the AF area

I does not happen every time, and is worse for small subjects

If a bird landed on the plants I would definitely miss focus on it.

If I change to C-AF things are much better, usually after a bit of trying the camera gets focus.

Settings are S, 1/1250, iso auto, AF scanner on, AF area single, (same with other AF area sizes)

Lens PL50-200 +1.4x (its the same without the TC)

So it looks as though S-AF is best for the specific task of picking out a bird in a bush (and I could set that up as my C1 setting, and C-AF for everything else with C2 for normal perched birds and C3 for BIF.

If anyone is still reading DPR, any suggestions?