6th Scarab slot not unlocking

Hi,

This is my first proper league so I may be missing something but from what I can see the first extra scarab slot is unlocked by completing a 10-boss Maven invitation and the second is from completing a T17 map for the first time. I have done both of these multiple times and still only 5 scarab slots.

I actually got 5 scarab slots on both trade league and a private league doing the opposite method and then when I migrated the private back to trade league I still only have 5 slots. I have since completed more T17 maps and done more 10-boss Maven invitations but it hasn't unlocked the 6th slot still. Am I doing something wrong or is this a bug with the private league migration?