[Not cancelling visa before leaving Thailand]

Hi everyone. My university will extend my Non Ed visa for an extra month as I am uncertain of the date of my flight. However I'm pretty sure that my repatriation flight will take place before the extension ends (May 31st, 2021). The school officer says that she cannot provide documents for visa cancellation simultaneously.

She says that I can resolve this issue (cancelling the visa) the next time I come back to Thailand. I heard that there are a lot of issues if one wants to come back later, when one doesn't cancel their visa before they leave the country. I am left with no choice. What's the worst thing that can happen to me next time I come back to Thailand having my Ed visa uncancelled? Thank you!