Well color me stupid or at least ignorant....
Next time I blow away any halftracks in the vicinity.
Here's a formal answer from Jason Petho:
My Sherman tankers may face court martials if this ISN'T a bug.
Playing Kucher's Ragtag Circus with version 2.0. Sherman tanks op fire is set to fire at hard targets only at long range. Soft and other targets are turned off. A german unit fires at them doing no damage. The Shermans expend all op fire at an empty halftrack. My opponent seeing this brings in his Panthers and lays waste to the defenseless Shermans.
Note that unit settings are identical for all of the Sherman platoons including the HQ platoon.
Unfortunately, since you mentioned "empty halftracks", one can assume they did carry infantry at one time, making them armoured. Meaning, they fall in the armour class, which means your OP-FIRE did what it was supposed to do.