Team builder has it's own MMR, much like normal queue unranked does. These are completely independent MMRs which are not impacted by any other mode's MMR.
Had a shyvana that rushed homeguard merc treads and botrk against a warwick and lost, then during teamfights, chased kassadin.
Blamed ADC for getting locked down, along with our AP trist which actually did good and our jungle Vi who assisted in chasing kassadin.