As long as they are in near perfect condition, no problem, however I did have a problem once in Patong, I was issued the £50 along with many others by the Midland bank in the Uk, the exchange teller said it was fake, within minutes two policemen arrived, I was asked if I had more £50 pound notes, I said yes in my hotel room safe. I was escorted on the back of a police motorcycle back to my hotel, where they inspected every single £50 note. They said they were all good and left. They even gave me back the “fake” and told me not to try to cash it anywhere else and return it to my Uk bank. I did, my bank said it wasn’t fake and credited my account £50 🤷♂️🤷♂️🤷♂️
I have never been asked at immigration about details of my return flight, but my passport is nearly full of Thai entry and exit stamps, I never go anywhere else 😂