By accident a came across the Thumbnails folder in the Shotcut folder. The images in there do not delete when Shotcut is closed, nor when the laptop is rebooted. Is there an auto-delete function somewhere?
They are not supposed to be deleted simply by closing the application or rebooting. The folder also contains waveform data. The code automatically deletes old data beyond 5000 files. You can delete them yourself, and they will regenerate as needed.