So, after investigation, for a given node, when there is a missing, it's because getExecutionReports doesn't return any reports.
It looks for reports AgentRunId(NodeId(a8f00955-7a4c-4e1e-8578-f4ef7909fcc0),2021-08-03T11:13:45.000+02:00)
But there is no run in ruddersysevents with an executiontimestamp of 11:13:45.000+02:00 for this node
8940 | 2021-08-03 11:13:46+02 | a8f00955-7a4c-4e1e-8578-f4ef7909fcc0 | inventory-all | inventory-all | 0 | inventory | None | 20
21-08-03 11:13:46+02 | result_success | Inventory | Next inventory scheduled between 00:00 and 06:00
even considering executiondate, it is
8928 | 2021-08-03 11:13:46+02 | a8f00955-7a4c-4e1e-8578-f4ef7909fcc0 | run | rudder | 0 | start | 20210802-174301-fafcc7a4 | 20
21-08-03 11:13:46+02 | control | Common | Start execution
Yet, in reportsexecution, there is this timestamp, which causes the error
a8f00955-7a4c-4e1e-8578-f4ef7909fcc0 | 2021-08-03 11:13:45+02 | 20210802-174301-fafcc7a4 | 8928 | 2021-08-03 09:13:48.422369 | 2021-08-03 11:13:52.208357
other node don't have this value either