Custom Image sizes wont delete

I didnt include a link because this is happening on a lot of my sites so I thought i would ask the question first an if needed i can provide a link.

Its as simple as it sounds – i have an older size that was created for some other purpose – lets its called ‘medium-large’ – its 1000×750 and crops – I dont use it any more and want to remove it – so i hit delete and it disappears from the Image sizes on the media settings page for the moment. I scroll down to save the changes and when the settings page finishes saving and reloads – BOOP – its still there. This has been going on for YEARS – can anyone help?