SDM or what?

I periodically run my SDM lenses to 'exercise' them, and everything has been fine...until today!

I put my 50-135 on my K5II, and it wouldn't focus! My 17-70 and 300 would, as did my new DC 16-85.

Oh ********! I thought. Classic SDM failure, but...

With nothing to lose I tried the 50-135 again. Worked! Then I noticed that it would focus from far to near, but not near to far.

Odd!

Tried the 50-135 on my KS1 and K20D. Focussed first time, every time, far to near, near to far, continuous AF - all perfect!

Must be contacts. Cleaned them on the body and lens (wiped with paper hanky) .

Tried lens again. No difference. Still not playing nicely. Tried other SDM lenses on K5II - perfect.

So, whilst it seems like the answer is 'SDM', I've never read anything that describes symptoms like mine.

To recap, the lens will focus perfectly on two out of three bodies, but only far to near on my K5II. All other SDM and DC lenses operate perfectly on the K5II.

Suggestions anyone please?