Okay, so I redownloaded the styles for New Formats, and found that Gronkling's Ancient tileset has actually already been converted - however, it has been split up into two graphic sets: "Angel Island" and "Chaos Angel".
This makes conversion of Old Formats levels using the Ancient tileset impossible, because there is no such thing as an Ancient graphic set anymore, even though the same pieces of terrain are used.
I did start making a conversion of Ancient myself, and with that the New Formats editor is able to read Ancient levels perfectly fine. But of course, it would be a bunch of duplicated pieces if I were to upload that.
Are there any specific reasons for the split of Ancient into two graphic sets? And if so, would you mind a "complete" Ancient tileset?
Otherwise, no Old Formats level using Ancient will ever be able to be converted. :/ And as I've stated repeatedly, for Lemmings World Tour, Ancient may just be the most important graphic set of all...