Sounds like a normal pawn capture to me. Do you have an example game where this happened?
Illegal "En Passant" move very common

You have posted the same question in three forums, and I think you have been given your answer, it is en passant. If you have an example that proves otherwise, then please upload. However, I feel it is very unlikely that the computer is playing illegal moves.

[Event "Computer"] [Site " Chess.com"] [Date "May 18, 2017"] [White "Chestplate2"] [Black "Comp"] [Result "*"] [WhiteElo "--"] [BlackElo "--"] [TimeControl "--"] 1. Nc3 c5 2. d3 Nf6 3. Bg5 d5 4. Qd2 d4 5. Ne4 Nxe4 6. dxe4 Qb6 7. b3 e5 8. f4 h6 9. Bh4 exf4 10. Bf2 Bd6 11. g3 fxg3 12. Bxg3 O-O 13. Bxd6 Qxd6 14. Nf3 Nc6 15. h4 a5 16. Bh3 Qg3+ 17. Kd1 Bxh3 18. Rxh3 Qxh3 19. Kc1 a4 20. Kb2 a3+ 21. Kc1 Rfe8 22. c4 dxc3 * Sent from my Android


so it finally happened mykaze1 and truffz. sukit! I read "En Passant" and my pawn did not clearly move two spaces. the opposing pawn was on the 5 rank, however (Jerks for implying my falsification), however my pawn did not move 2 spaces forward, And the opposing pawn made the capture well after my pawn only moved One space and not two, clearly not satisfying the condition that the capturing pawn can ONLY make the move after the opposing pawn moved two spaces forward in one move. So yes, The computer malfunctioned. Yes you are wrong, yes you wrongfully implied that I lied, yes i own the right to say, "suck it be-otch," because you unfairly failed to yield me the benifit of the doubt! can you tell I took your comments rudely and insulting? Maybe some lessons in manners are in order......

https://en.m.wikipedia.org/wiki/En_passant

You said in one of your other threads about this same thing that you studied up on 'en passant,' but you apparently didn't quite grasp it completely.
When you moved to c4 on move 22, you moved it two squares up, from c2.
If you move your pawn up two spaces in one move to where it lands right next (side-by-side) to one of your opponent's pawns, then they can take it as if you only moved it up one space.
However, if they choose not to do so immediately, then they don't have the option to do so on their next turn.
https://support.chess.com/customer/portal/articles/1444779
(Edit: the link 1818 posted above this comment is a much more detailed source -- i recommend you visit that one rather than this one.)
Many times has my chances of beating the computer on max level because of this illegal move. The computer executes the move when the pawn is beyond the 5th rank AND at times when my pawn hasnt foved the two square move to the 4th rank. This SO FRUSTRATING!!!!!!!!!! PALEASE FIX THIS!!!!