Theme Preview Link Shows Live Site, Not the Preview

[TiVa-23]

I have had this problem the whole time I have been using Shopify (about 1 year), where when I try to view the preview link for a unpublished theme I am editing, I see the current live site, NOT the theme I am trying to preview. I have tried everything suggested in other posts that mention this problem, including clearing cache and cookies, using incognito window, using a different browser, or opening the link on different devices. The link always displays the live site, not the theme preview. 

Has anyone ever found a solution to this? It's VERY frustrating not being able to preview a theme on my phone or other devices before publishing it.