MacBook doesn't remember Screen Orientation
I had this same issue. So the solution is:

The external monitor should be connected.
From App drawer open ColorSync Utility.
In Devices column, select display and Under Displays select your external monitor.
Check if &quot;Factory profile&quot; path and &quot;Current profile&quot; path both are same.
If not select drop down arrow next to &quot;Current Profile&quot; and select &quot;Other&quot;.
Select the same file name which is mentioned under Factory path.

Now your monitor will remember the screen orientation. Hope it helps :)