Steps to reproduce:
1) Open initial board position
2) e2-e4
3) FEN changes, but EP Target Square is missing
imgur.com/a/K7quv8k
Specification (Standard: Portable Game Notation Specification and Implementation Guide):
16.1.3.4: En passant target square
"[...] Therefore, an en passant target square field may have a square name even if there is no pawn of the opposing side that may immediately execute the en passant capture."
1) Open initial board position
2) e2-e4
3) FEN changes, but EP Target Square is missing
imgur.com/a/K7quv8k
Specification (Standard: Portable Game Notation Specification and Implementation Guide):
16.1.3.4: En passant target square
"[...] Therefore, an en passant target square field may have a square name even if there is no pawn of the opposing side that may immediately execute the en passant capture."