If the dashboard created in 2018.1.5 relies on features that were not available in Tableau Desktop 10.5.3, they will be removed from the workbook when you downgrade it. Did you use any features new to 2018.1, like step and jump lines, in your dashboard?
That's a good question. I'm not sure but I believe they may have.
So when we downgrade to 10.5.3, I noticed we get NULL variables for our data, causing the rendering of any data points in our graphics (maps, etc) to no longer appear. I got around this once by refactoring the data with the original Excel files used to build the workbook, but we weren't able to apply this resolution across the board with all of our users.