Banners disappear, even when in valid spots

[theTrav]

I just had 2 more banners vanish when I completed them.

For the second one I tried re-placing one in the same spot and the construction site allowed me to do so.

To me that indicates that it's not a matter of the site becoming illegal, but something else.

The construction sites were both quite old

[Asmodeus]

I have had the same problem when finishing a banner someone else started goodness knows how long ago out front of brodgar.

[Laremere]

Banners should drop their building materials if they aren't valid anymore.