action is cancelled due to ancestor's failure yet the ancestor completed well.
The error is misleading. It says "Cancelled due to ancestor's failure" but this error also occurs when a loop that starts right after the ancestor has 0 iterations, i.e. the actions within it do not run at all.
Release Automation 5.5 and higher
Find out why the collection elelement or xml file that the looped flow is based on is empty. Then correct it so the flow is executed at least once.
A common cause of an empty artifact related collection element is that the artifact definition is not mapped to the same server type as the process that runs the flow.