I've had seemingly the same issue. After several weeks of running a backported apache2 2.4.37-1 the issues are gone.
Previously it was happening several times every day, with the outage lasting sometimes 10 minutes or so. This was very difficult to troubleshoot, as nothing is logged, and it was relatively hard to find this bug report and find my way towards a solution. Applying the fix to stretch might help others who are struggling to understand the issue they are facing and are therefore not being heard.
