Completion broken… Hum, looks like the shift key is cheating on me:
Typing "Op<TAB>" in xev(1) shows:
Shift_L Down
O Down
O Up
Shift_L Up
P Down
P Up
Shift_L Down
ISO_Left_Tab Down
ISO_Left_Tab Up
Shift_L Up
It turns out the shift key is a bit sticky and does not raise completely as soon as I release it. When I press Tab and it is only half-raised, the #typematrix skin deformation is strong enough to press-down the Shift key again and the keybaord sens a Shift before the Tab key is down.