Then to clarify:
A flag would be dependent on a new action such as leaving a block (after loss of helmet) to engage in another block or, if on defense, to chase down the ball carrier after being blocked;
or,
If engaged with a loss of a helmet during the engagement, the deciding factor would be the length of the engagement (time) after the loss - a short time would be no foul; an extended time, even if engaged with the same player, would be a foul.