What strategy does MySQL optimizer use when choosing between two indexes?
Index selection is based on the estimated number of rows that will be accessed when using the different indexes.  The order the indexes is declared in should only make a difference if the estimated cost is the same for both indexes.  In your case, the three columns org, country, and author_id can be used for index lookup. Any index that start with these three columns may be used and will have equal cost.

The most likely reason for your observations is that the InnoDB index statistics have not been updated after the bulk insert.  If I do ANALYZE TABLE on the two tables, the optimizer selects i0 index instead of author_id.

If you want to understand more about why a particular query plan is chosen, I recommend to look at the optimizer trace for the query.