MDEV-11369 implemented instant ADD COLUMN, it would refuse to do instant ADD together with ADD INDEX.
Adding an index to an instantly added column does not make much sense, because the index would be useless due to every row initially having the same value for the instantly added column.
However, it is meaningful to ADD COLUMN together with ADD INDEX on some old column. This will currently cause the table to be rebuilt:
The first ALTER TABLE should be fixed to allow ALGORITHM=NOCOPY, but the second one can in my opinion continue to rebuild the table with ALGORITHM=INPLACE.