We had kind of the same (nightly after a cron and yes apache hangs) years ago once a week, SMTALK did solved it but was also a kind of bug , i can't find history, but even if i was not told what the real cause was , only a DA update came out shortly after it.
Not helps you but, i can remember however together with the solution a custom apache was running after it because we wanted HTTP2 and newer TLS version for it. So 2 things in one work the "BUG" and setup custom HTTP2 ( the manual for that was then not working for me, maybe i did it wrong).
It wasn't possible to restart apache then, a reboot was needed , i did found out later wtih htop how and which id to kill manually