InnoDB multicolumn fulltext search: SELECT MATCH requires single column index?

Question

I have table with two fields and run a fulltext query with weighted order:

SELECT 
  MATCH (`my_table`.`text_1`) AGAINST ('+lorem' IN BOOLEAN MODE) AS `reltext_1`, 
  MATCH (`my_table`.`text_2`) AGAINST ('+lorem' IN BOOLEAN MODE) AS `reltext_2`, 
  my_table.id
FROM 
  my_table my_table 
WHERE 
  (
    MATCH (
      `my_table`.`text_1`,
      `my_table`.`text_2`
    ) AGAINST ('+lorem' IN BOOLEAN MODE)
  ) 
ORDER BY 
  (2 * `reltext_1` + `reltext_2`) DESC

There's one multicolumn index containing both fields text_1 and text_2. This was a MyISAM table in the past and everything run fine.

Now when I switch the table over to InnoDB I see a Can't find FULLTEXT index matching the column list error. What I figured out is that everything works again if I add two additional single column indices for text_1 and text_2. Is this really necessary? Do I do something wrong?

Edit: Tested on MySQL 5.6.38

Answer 1

Alas that is one of a few differences between the two separate implementations of FULLTEXT.

Alas. the workaround is to add two more indexes:

FULLTEXT(text_1)
FULLTEXT(text_2)

But... Don't count on relevance numbers coming back from different indexes as being comparable.