I am already sure 99% that FIFA 19 is scripted but my experience has been worst with SBC, TOTS Challenge, Upgrading players, exchanging squads...
In my case I am guaranteed to get a GK. I got only GK from TOTS Guarantee SBC: Trapp 95 , Benitez 93, Oblak 95, Onana 88, Sirigu 93, Koval 84. At this point I don't believe that Rewards aren't under control of EA.
Even when I tried to exchange squads for Flashback, UCL upgrades,... GK again and again!!! I got De Gea 92, Casillas 91, Handanovic 89, Adan 84, Akinfeev 81... I could build a team with all my GKs.
Could you please unmark that GK is my preferred rewards!!!