I have spent many an hour in ULS viewer, shuttling between normal and verbose logging tracing down correlations to sometimes happy conclusions. Since Microsoft does not allow tenant administrators into the back end, I would have supposed they would do this. The workflow in question is 2010 in origine and has been in continuous use since 2016 without issue. Suddenly a user reports issues, normally they get an email to let them know the workflow ran to completion.
This policy of non assistance on ULS logs in SAAS O365 I consider quite rude and totally unhelpful. Generic advice won't help in this instance, but a brief look at a correlation with an exact timestamp and tenant id might.