As an example I have 5 Tempo worklogs on an issue, 3 are billable and 2 are not billable.

Each time a log is entered the following automation is triggered:

I order to better understand the issue I've added a comment for each billable worklog:

The custom field has at the end the last iteration value, instead of the needed sum of 6h.
I've tried to get this running with updating the issue field (triggerIssue as well as issue) and getting the value again via web request in order to increase it. I've tried the same now via a project property. But in none of the cases the value is updated withing the loop. It always remains within the loop with the original value that it had outside the loop.
How can I get an increasing number field that modifies the outer issue custom field (JQL not possible with TEMPO)