When entering a zone ( own for friend island ) and activating a buff ( basket, feast etc.... ) buildings that can be buffed are highlighted as before.
When buffing a building highlight is 'turned off' - untill buff is activated, then it is highlighted again if buff is still selected.
Result is that all buildings that can be buffed with selected buff and was not already buffed when entering zone are highlighed - both newly buffed or unbuffed.
If leaving zone and returning buffed building shows correctly.
I think it is a bug - but maybe just another unannounced feature to make buffing harder.