When you set the mega menu sub item type to default, you have access to the dropdown max width. Whatever I put in this field, the menu drop down width is still the equal to the menu width, just like in container mode.
Thank you for contacting POSIMYTH Support.
Can you please share the site URL and wp-admin login details so that we can check from our end?
Warm Regards,Deekshit Nair
I'm located in India. My timezone is IST.