It did happen again for me. I’m still on FreePBX 16 on my main system (no time to update to 17 right now…).
I attach pics of the crontab on my system in the following order:
- When I noticed the issue, the crontab only contained one line (the Job worker)
- I updated some modules (system admin and framework) and some lines returned (even though they are not certman related, and this confirms the theory that this is a system-wide problem)
- After a force reinstall of certman, the crontab entry is restored
I point my suspects on the Job class 
maybe it is periodically purging the cron entries?