Some preview versions of NTT have issues with replacing some sprites. There is no apparent dependency, but it's different sprites per version, leading me to assume that it's some issue with texture pages. I hope to either workaround the issue by the next public release or migrate the mod onto a version of GameMaker runtime that does not show the issue. In the current-most version /loadsprite includes a workaround, but /loadimage doesn't, therefore /savesprite'ing the sprite and /loadsprite'ing it back (replacing the base sprite with a same-looking imported one) before calling /loadimage fixes the problem.