AFAIK, I could barely see MOTD open just before getting disconnected.
In log you can see "user entered ingame / disconnect deltaEnt" the same second. Maybe it's something like "full-update" event or something like this which is overflowed.
By the way, when a deltaEnt issue happens, none of the already connected client get disconnected. Even if you are minimizing the game to force a full-update. This bug exist in CSGO engine since 30 april 2015 (version 13482). Map with too many entities are broken, even if sourcemod is disabled.
Maybe it's possible to disable this breaking event (fullupdate?), or to shrink it. This reminds me this plugin
https://forums.alliedmods.net/showthread.php?p=2108509 for a similar issue. I hope someone will find a way!