It's not a bug. You just misunderstand how a sequence of if statements work. If there are no other lines in the program between if and otherwise statements they are taken as one logical block. This is necessary, to be able to handle multiple possible cases, with a final "otherwise" to catch any cases that are not explicitly handled. Note the difference:
You are not the first person to be confused by this. The way the program is written is rather ambiguous, and relies on the reader to know what this structure will do. It would be preferable if it were obvious to the casual reader. That is exactly why the syntax in Prime was changed. In Prime you have "if", "else if" "also if" and "else" statements, so there is no confusion about what the program will do.
