Illegal substitution?

[drs]

Team A runs a no huddle offense but not a hurry up offense.

After each offensive play, a 12th A team member runs onto the field just inside the numbers. As the offense looks over to the bench for the next play to be signaled in, this 12th man may stay on the field and another player may exit, or the 12th may come back off the field, depending on the formation or package that they want on the field for the play signaled in.

Did the entering 12th become a player? I think the only part of 2-32-15 that could apply is if we consider that he was positioned in the formation. If he is, then we know we have a replaced player due to 2-32-12 and we can now hang our hat on the 3 seconds in 4-7-1 or 4-7-3 if he comes back out.

I am curious to see if everyone considers the 12th as having established himself as a player in this situation?

Thanks in advance for your replies.