The rx enforces the sequence goes up.
You press button to open. Attacker lets the first sequence go through and the door opens, while the button is still down the attacker jams your second transmission while capturing it themselves.
Now they have a code they can use to open again when you're not around, assuming you don't use it again in the meantime.
If you wonder how vulnerable systems keep getting deployed without it being malicious, you don't need to look any further than the nearest hotshot that thinks everything is "not that difficult" and that everyone else is incompetent.
Security of any kind is just hard. The defender must defend against any possibility while the attacker needs just one vulnerability. How much cost and range and battery life are worth losing when the attacker can just punch through a window with their fist?
