Any chance you could post one or two of those incidents?
I figured it out, Lower hull Shots beyond 1600 yards will result in a kill for an M4. So Panthers are useless from the front unless front lower hull is hidden.
Beyond 2k yards a sherman can't kill a Panther via front lower hull. What I found out is the upper hull and Turret are immune to an M4, however the Lower hull seems to take shots up to 2k without any problems killing a Panther.
Something seems weird about a round flying beyond 1700 yards, to penetrate a panthers Front Lower Hull especially on the angle it has?
Edited:
I ran the test with Hooter in the DA, an M476 at 1400 yards (upper hull, turret, then lower front hull)
Each time up to 2k the Panther died via Lower front hull, now turning 10 degrees to either side from a head on position, the panther did take a few rounds extra, but still died at 1700 yards.