Hi Tristup,
I have had the same thing happening in three different browsers: Chrome, Safari and Firefox (developer edition), all on an M1 MacBook pro. Chrome is on version 94.0.4606.71, Safari on version 14.1.2, and Firefox on version 93.0b9
I know that normally the generated CSS overrides the stack CSS, but in this instance, the generated CSS for the Line element only (as far as I can see) is being overriden.
Further to my initial report of this issue, here is a more exact description of the effect of this bug:
- On loading the Page Builder, all Line elements show exactly as expected
- Changing any style control of any element in the page causes the Line elements to disappear (because of styles being overridden, as shown in my previous screenshot)
Using Inspect Element on the Page Builder iFrame, I have found that when the Page Builder is first loaded, there are three CSS rules contributing to the border style of the Line element. The first two shown below come from x-stack.min.css.

The third, which overrides the first two on first load, is the generated CSS of the Line element:
Then when a change is made to the Line element, and the Line elements in the Page Builder all disappear, there are five CSS rules contributing to the border style of the Line element. The two extra ones now come from integrity-light.css, and it is the .x-line rule from this CSS that ends up overriding the generated CSS.
Looking inside the <head> element of the Page Builder iframe, when the editor is first loaded, the <link> referring to the x-stack.min.css file is above the generated CSS, but when a change is made, a new <link> is added below the generated CSS, pointing to the integrity-light.css and loading in CSS rules that override the generated CSS.
I hope this helps in recreating the bug on your end and fixing the issue.