I think wic is adding the GPT copy at the end, but due to various padding values being subtracted from the total size of the image, the final size is not 100% accurate down to the sector count. So the image ends up being just a little too small, and therefore the GPT copy is not found by the kernel in the expected location. In general this message is harmless, and is a weakness of the GPT format (strange that they made the validity of the GPT table dependent on the size of the storage medium).
This would be fixable by making the Mender image generator sector accurate, but since this has no ill consequences, and only wastes a tiny amount of space, no one has stepped up and fixed it so far.