Could be that it has to do with where the ammunition is stored. In a panzer there would be rounds in the turret, and I presume this is the reason you can kill the panzer with frontal shots to the turret. But the hull from the front seems to be too thick for the M-8 to penetrate. Osti's and WW's have the same hull as the IV, but perhaps there isn't enough ammo in the turret to brew up the tank?
Oddly, though, I was able to find a "sweet spot" on the M4A3(76)'s front turret and could kill it (shot trap, maybe?), but could not find a similar sweet spot on the M4A3(75)'s front turret. Hulls on both could be penetrated in the lower rounded portion that does not afford as much slope as the upper hull.
Even more odd, the T-34/85's front turret armor seemed no problem for the M-8 to penetrate, although the T-34/76's turret seemed invulnerable. The 85's turret front should be a small bit thicker than the 76's. It is nearly twice as thick as the panzer's turret front, but it seemed the 85 died quicker than the panzer to front turret hits.
It is also thicker than the panzer's front hull armor (90mm vs. 80mm), so unless HTC is modeling different quality armor (which they I suppose they could), one would reason that an invulnerable panzer front hull should also mean the 85's turret front should be invulnerable (or the panzer's hull made vulnerable).
