Fascinating. These rules are identical, but I don't have xfractint locally to reproduce/troubleshoot the issue. I think I was using https://dmitrykandalov.com/lsystem/ as a playground, and it appears to be fine there.
Can you try generating 1/8 or 1/4 curves to check the partial generation? Or these rules--should produce the triangular shape:
Can you try generating 1/8 or 1/4 curves to check the partial generation? Or these rules--should produce the triangular shape: