Lowe may have moved, quite honestly I did not see it. What I did see however, was no flag nor whistle until the Purdue defender kept pointing and waving.
I never understood why an illegal defensive substitution was blown dead before the play. Not too long ago it was like a free play, similar to offsides. Let the offense have a chance at a play, then enforce the penalty or the result of the play.
No reason to stop a play, especially when player is running off the field ( most of the time.)