I have to admit that the mention of "Sandy Island" baffles me. I went to Wikipedia to find out about and discovered there are about a dozen very real places called Sandy Island. If there was an "imaginary" place, somebody should be more specific about which maps had it and where. I will point out that some reefs, atolls, and islands pop up or disappear over the course of time, as for example the result of avulsion, erosion, hurricanes, etc., so the mere fact that an island was mapped at some point and later not found (or did not appear on old maps and now is mapped) is not proof of an error.
The point that there is not an "official" Flat Earth map is very telling. It's not a question of "official". In the matter of globes that supposedly represent a Round Earth, there is no "official" globe - not the Replogle, nor Rand McNally, nor even National Geographic - but all the globes closely resemble each other in layout. If the Earth is really flat, then a map of it on an equally flat piece of paper should should pretty much show the layout with no distortion in shape or directions and the only math involved is the scale (say, one inch equal ten thousand miles) .... and every other map on flat paper of the flat earth would closely resemble each other. No such agreement in FE maps. The excuse is they're not "official". But a map of the Flat Earth, to be accurate, should show where The Edge is - and it would be in every direction if you go far enough; it's not even clear if The Edge is a perfect circle or a rectangle or a square or a polygon or something with jagged edges. Nor where it is, at any point at all; what is clear is that it positively cannot be in Antarctica because plenty of ships have completely circumnavigated Antarctica.
Now it seems to me that PROVING that the Earth is Flat requires a map that is, if not perfect, at least fairly accurate about such a crucial detail as the location of The Edge. That the FES is still using a 19th century map (based on 17th century math) in the 21st century, with absolutely no further information about the location of The Edge, is Very Revealing.