I have created a Tableau dashboard (with both Mobile and Desktop layout) using Tableau Desktop (2018.2.1) and published it on the Tableau Server.
If I open this dashboard on my monitor (1920 x 1080 resolution), It opens the desktop layout, and if I drag this web browser window from Monitor to Laptop (1920 x 1080 resolution), refresh it, then it shows the default layout which has both the Mobile and Desktop layout.
How this dragging window from Monitor to Laptop confuses the Tableau to show Default layout when both the screens are Desktop layouts only?
Have you tried publishing the workbook without "Show sheets as tabs" checked? (as I think then the dashboards will retain their individual sizes)