Siteground likely has a plugin of some kind that filters the return values when using WP native image functions.
Images uploaded through an ACF field go through the same process and images uploaded anywhere else in WP using WP functionality to do the work. If this conversion happens on upload then there should be no reason that it would not work with an ACF field.
This is something that you need to talk to siteground about and see if they have any suggestions.
