Mail Split View not working in Mac OS Golden Gate

The option of 'Prefer opening messages in Split View when in Full Screen' doesnt seem to work in Mac OS Golden Gate.

When I open Mail in Full Screen mode and then click on new email compose button, a new window emerges from the bottom of the screen, rather that the mail splitting the screen to accommodate the same window on the right side. Please help!

Mail Split View not working in Mac OS Golden Gate
 
 
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