Screenshot.png Not displaying on admin

Hi, I have installed a premium theme and changed the screenshot.png and uploaded it via ftp, upon looking at the admin panel it still display the old screenshot.png and the link it was linking was

https://www.mysite.com/wp-content/themes/mytheme/screenshot.png

and if I change the link to

mysite.com/wp-content/themes/mytheme/screenshot.png

it shows my new screenshot, why does the admin panel detects the screenshot with www and if I look on my hosting file manager my theme has the new screenshot. I’ve tried to delete and reinstall wordpress from scratch including the theme but still doing the same thing. any thoughts?

Thanks!