But nevermind I think I found solution in any case..
Hi I think what you did is switch from default editor to advanced layout editor.
That is not the issue, the issue is there ONLY when switching from text editor to visual editor and other way arround. Also it is not just src of images, it is also links of any link
well it happens no matter if page is saved or not, it always removes img src when changed from text to visual or from text to visual and back.
Adding img src to any of the text boxes (including content slider) removes src from images but keeps classes intact.