The defrost drain under the evaporator is blocked. Remove the back panel in the freezer and pour very hot water into the drain pan to melt the top layer of ice, then probe the drain hole with a small screwdriver blade until you work the clog loose. Follow up with a cup of hot water and a capful of bleach to kill any residual algae that may be in the line.
On this model be sure not to turn the freezer temp control above the halfway mark. This tends to make the drain freeze up less. Also, check behind the middle crisper drawer on the FF side way back on the back left wall where there is a plastic return air duct clipped to the corner. This duct cover must be in place and the clear plastic film that is inside of it must be intact and not melted or damaged. Sometimes whe the adaptive defrost boards would fail on this model, it caused this cover to melt or degrade. This in turn caused air backflow issues in both sections, even when the fan is not running.