What is a password…?

So I reset my password on my Wi-Fi through the app as well as online (verified/checked). I unplugged my router from the wall and outlet to reset the router.  I then plug it back in hook it back up and go to connect devices(ring/fire sticks/tablets) upon connecting and entering new password it states password incorrect. So I put in old password and it connects. Is there a time frame on when the new person takes place of the old to connect to devices or is this just a common problem? It’s weird because I’m resetting my password due to the suspicion of others connecting to my router yet the new pw isn’t working and the old continues to go through.