Yeah... the '\' character is treated as a line-continuation character if it appears last in a line. Nasm needs to "get" a line from the file before it can "parse" the line to see if there's a comment in it, so it isn't possible to ignore it after ';'. This is intentional behavior, although it's inconvenient if you're trying to do "ascii art" in a comment. If you don't want '\' to be treated as a line-continuation character, put something after it. Even a space will work, although that might be confusing. This broke some existing code when it was introduced, which we try to avoid, but we really needed a line-continuation character...
Best,
Frank