I had 7 slots for characters prior this update/server merge. (SA gienah)
Today when i logged in my characters were in the storage as intended but its not displaying the extra slot that i bought.
What can i do to retrieve the missing slot? Is there any official news about this issue?
Buy a new one 
Go into the supportchat
Some friends of mine had this question too, it seems they locked the slots but gave the unlock tickets back. You just need to use them again, clicking at the locked slot.
Hey there @M2Punk!
I understand you have questions about your character Slots.
As mentioned in the Server Merge Details post: Character slots will reset to the default 6. Additional character slots that have been purchased will be returned to the player as Character Slot Extension Tickets.
Meaning your extra slot was returned as a ticket, so please let us know if you got your ticket back and if you’re able to use it. I’ll be around in case you need help with anything else.
(Thanks by the way @Shangyue for helping our friend on this one) 
Thanks for the help guys, found it!