Substitution.
 

NFHS rules - Team A requests and is granted a TO. After TO is over A1 enters the floor but before the ball is put in play leaves the floor because he believes he has been substituded for. After ball is put in play for about 1-2 seconds A1 then runs onto the floor and promptly receives a pass. A1 was an original player before the TO request was granted. Are there any issues with this? I am thinking there is and that once the ball became live he couldn't enter the court till the next stoppage of the clock. Is it the same for NCAA?

No issue in NFHS, as long as the delay in returning to the court didn't provide an unfair positioning advantage. 10.3.2.B

Even if A1 originally came on the court at the end of the TO, if s/he left before the ball was made live then I don't think s/he "returned at the same time". T (by rule) in NFHS.

If this was after a substitution, then it's legal.

Legal either way (I think) in NCAA(W).

...Or that. 10.1.9