Mooshrooms disappear with shears

Given the structure at attachment 0 (the preassure plates and repeaters are for testing purposes), if we spawn there a mooshroom (attachment 1) everything is fine.
From a distance of ~5 blocks horitzontally, if we shear on the moosh, it will produce the animation (attachment 2), but the "resultant cow" has disappeared.
The sounds of the cow remain there, as the cow in fact IS there (attachment 3), so if we shoot the cow with a bow, it will die (attachments 4 & 5).
The collision system fails to work. If we stand in the place where the cow should be, we are not pushed.
If we exit the world, and reopen it, it reappears.