MySQL full text index doesn't show up in query plan when use with multiple match and related tables

[Ajith PN]

explain

select distinct o.id, o1.id, order_items.order_id
from orders o
left join orders o1
on MATCH(o1.po_number)
AGAINST('+test*' IN BOOLEAN MODE)
and o1.id=o.id
left join order_items
on MATCH(order_items.product_description)
AGAINST('+test*' IN BOOLEAN MODE)
and order_items.order_id=o.id
where COALESCE(o1.id, order_items.order_id)is not null

In the above query plan the full text index is not present in the possible keys. Can anyone help with what is wrong here? Thank you.