Hi,
Just posting this here as I spent some time trying to solve this.
After upgrading I had an issue where cron was running, but the daily automated tasks were not, Automation Overview was all zeros and System Health was alerting me that daily tasks had not run.
If I ran cron from the shell, it exited with no errors (RC=0).
Eventually I tried running it with the --force option and then I received a php error indicating that it could not find bcmul.
Turns out that WHMCS requires the php bcmath module, but does not tell you this anywhere and it is only when you call cron with --force, you see the error.
Anyway, if you are having a similar problem, install php-bcmath and you should be OK.
Hope this helps someone and WHMCS Devs, if you're reading this, please make php-bcmath a requirement and alert on it in System Health.