Your software
My Mautic version is: 4.2.1
My PHP version is: 7.4.28
My Database type and version is: 10.5.10-MariaDB
Your problem
My problem is: Cron job updating segments will not proceed if there is a problem with one segment. For larger systems, this should be more robust.
Steps I have tried to fix the problem:
Create 3 segments, a,b,c
Make the filter on Segment b, dependent on membership in segment a, and some made up tag.
Now delete segment a.
When you run the cron job to update segments you will get a SQL error and processing will stop and not process segment c. (because there is no segment a, which creates a blank element in the SQL)
The segment update icon (!) does not show up for some time. Then you have to run the cron manually to figure out which segment caused the problem, then diagnose the issue. Then re-run the cron job.
It seems the best way to handle this is catch the error from the bad segment and then proceed to the other segments and update them, then the update icon (!) would be showing the one segment that has an issue and be easier to diagnose.