Ok AgitatedDove14 SuccessfulKoala55 I made some progress in my investigation:
I can exactly pinpoint the change that introduced the bug, it is the one changing the endpoint "events.get_task_log", min_version="2.9"
In the firefox console > Network, I can edit an events.get_task_log and change the URL from …/api/v2.9/events.get_task_log to …/api/v2.8/events.get_task_log (to use the endpoint "events.get_task_log", min_version="1.7" ) and then all the logs are returned by the endpoint. When using the 2.9 endpoint, no logs are returned