why does maintenance mode produce nginx 503 error for me in elgg 1.9+ ?

i need to put my site into maintenance mode and i just discovered that the problem i had in 1.9 where nginx throws a 503 error page instead of the maintenance page is still occurring for me in 1.12.4.

does anyone know why this is?

i haven't found any webpages on the topic or any useful info from any logs. :(