Steps to reproduce this bug:
  • Start construction of the floating residence.
  • Visit a friend's island.
  • Return to the home island.

The floating residence appears to be fully built yet still has a long time to finish.

I think also the annoying longbowmaker quest caused it to appear fully built too, but I wasn't able to confirm this as I wasn't looking out for it when I suspect it happened the first time.

It's definitly only a visual bug, no need to rebuild it.