Howard Duck places invis tiles only on (basic) purple tiles.
The ability description does not specify this limitation, thus creating less invis tiles sometimes.
If memory serves, there are some other characters with similarly color-restricted tile placement that don’t specify the tiles. I checked a couple, but it’s not the ones I was thinking of.
Yes, this seems problematic, especially when you have other abilities out there that overwrite tiles to place theirs. Classic Magneto for example.
This is pretty normal, though. Very few abilities overwrite other special tiles when they fire, and those that do tend to mention it explicitly.
I agree it would be nice if abilities were very concise as to how they work, but let’s face it; we’re not playing this game because of its history of elegant and well-templated ability descriptions (although Exquisite Technique is pretty cute).
Maybe it could have an if/then like IW, Moonstone, … if there aren’t enough tiles out, it creates them.