I have found that in many cases the preview of elements that have flex css properties is not correct. Sometimes it has to do with units relative to viewport (e.g. vh or vw), other times it has to do with text containers, such as paragraphs.(I have previously documented one such case)
The bug with viewport units is not only related to flex, but seems to be more general.
I will try to add cases and specific examples here when I find time.
I'm not 100% certain, but I think the "flex text container" bug appeared with the introduction of bootstrap version 4.
These bugs are rather important because they make the preview useless when there is extensive use of flex properties. For the time being I'm forced to use the browser as the only "source of truth" when it comes to layout visualisation and the preview window just for drag and drop.