11th Man Running On....

[OverAndBack]

Had this situation in an 8th grade game recently:

A's ball 2nd and goal at B's 6 yard line. B notices that they have only 10 defensive players. So as A is coming to the line and after the ready for play, B11 runs onto the field. He doesn't make it to his side of the line before the snap.

B2 runs 6 yards for a touchdown.

My partner flags it, calls offside (actually encroachment, there is no "offside" anymore, right?) and A declines it and takes the touchdown.

I ask my partner, "Isn't that a dead-ball foul?" He says, "No."

Now, if it's encroachment, doesn't it have to be a dead-ball foul (no play, no touchdown)? And if it's a substitution infraction, isn't it also a dead-ball foul?

A won by 34 points, so it was not the biggest deal in the world. But if it happens again, I want to make sure I get the call correct. What do y'all think?