One of my servers with apache 2.4.52 suddenly failed to accept connections. It turned out no child processes were spawned, only the main process running as root.
Seems to be other reported as well: https://forums.cpanel.net/threads/ups-421-apache-2-4-52-issues.696537/
Reproduced on second machine with 2.4.52 and MPM event.
Fix present in 2.4.53:
*) mpm_event: Fix a possible listener deadlock on heavy load when restarting
and/or reaching MaxConnectionsPerChild. PR 65769. [Yann Ylavic]
I've just added apache 2.4.53 to the tree, so this can now be tested.