Here are some other ideas:
1. Gently push the strings back and watch the motion of the damper. Does it follow the strings? If not--if it just hangs there without moving at all--it needs adjustment of the wire to give it more tension. But see below first...
2. Check the pedal adjustment. The pedal should have just a little lost motion (slack) before anything happens. However, occasionally some dampers might lift just a little earlier than others. If that's the case, over-tightening the pedal, even by a tiny amount, could hold those dampers open just a tiny bit. When I do a pedal adjustment, I always go through each note to make sure they all damp as well as possible. If one or two bleed through, very often I just back off on the pedal adjustment and that solves the problem.
3. Are either of those strings new? Are they wound? A new string could cause poor damping, which is why new damper felt should accompany a restringing. I'm guessing that's not the case here though.
4. In verticals, that last tenor damper is usually smaller than the others, just a tiny wedge to fit next to the plate strut. If adjustment doesn't work, there's another option:
an overdamper. Many Yamaha verticals have overdampers installed at the factory to help this last damper. It's a small damper that is added above that sits on a wire attached to the main damper block. If nothing else works, you or a technician might be able to fashion one.