The details can be found under Monitoring → Logs. Please look for the log with description “Update a Multi-factor Authentication Factor”. Under the details of the log, you may find the email of the user who made the change.
The strange thing however is that when selecting this filter, three items were shown in the search results (two of which were recorded yesterday) but when I click these items I get an error.
I reopened Monitoring, Logs again, applied the MFA Settings Update filter again and now I see only one item in the search result.