Why does Apache throw a 502 error when restarting with graceful?

Question

Apache HTTP Server

There is a powerful server and it has nginx+Apache (fastcgi) installed as web servers that serve several thousand small sites, mostly with minimal traffic. There are no well-visited sites. The load is normal. Once every 5 minutes, Apache is restarted with graceful to pick up changes in the config (add/remove/change domain configs).
The problem is that during the restart period for 2-5 seconds. an error 502 is issued, although, in theory, when restarting with graceful, this should not happen (it's not restart, when all connections are cut). Error 502 is issued not to everyone, but to some connections, but to a very noticeable part.
In which direction to dig? What could be?