Does 3 c4 no longer disable+kill tanks?

(PC) I don't know about you guys but today I have not gotten a single tank kill off of 3 c4, even when placed on the back. In fact, just now I found a completely oblivious nonmover and made SURE I planted 3 right on the back and I got only a disable.

Edit: Well, others don't seem to behaving the problem. My best guess is that the reactive armor perk stops tanks from dying in one hit with C4, otherwise 3 should always be enough.

What gives?