Using CSV Import can result in a message similar to "The record is locked by a Workflow". But Mass Updates got you to 100% complete, and the record are not modified at all (because of the blocking Workflow) but no warning, error or log otherwise. Is this by design?

most probably, because you have option to select workflow or script execution for the CSV import. Note that you if you exclude CSV in the workflow context and only trigger in create then csv import can update the record in theory

Yep, CSV Import works fine, I used it in the end. I tried Mass Update first because a very simple condition was needed to filter the records and a single field, with the same hard-coded value was the intended change, so there was no need to mess with CSV export > clean & set > import > run, etc. But alas, better to use CSV Import next time.

yup I concur - more control over all