Text-to-ASCII art depends on font choice and fixed-width spacing. Generate a small sample containing letters, a digit, and a space; compare the on-page preview with the copied result. Leading spaces and trailing line breaks may matter when the art is pasted into source code or a terminal.
Try a narrow destination and a second font to see whether the issue belongs to the font or the copy action. A tool may represent unsupported Unicode characters differently from plain ASCII; document that behavior explicitly. Include the input, font, output width, and the first line that shifts.