OK it seems like whatever fare you select for the outbound, you get for the inbound, so Standard is Flex + 50€, which is exactly what it would cost you to add one bag each direction (for non-elite PAX).

It is the same in the AF app, and AF website is not very consistent about enforcing the same fare for both legs, I guess we have to wait for KL website to update and see whether it is a 'feature' or a 'bug'