Nobody says "Bonjour" to say goodbye in French. Maybe "Bonne journée" instead, but not "bonjour". Being a French speaker, I have practically never heard it said that way. The only time (I think) I ever heard it was when an old person/boomer used it, but "bonjour" as a means to say goodbye is generally very archaic and outdated.
When someone says "bonjour", 99.999% of the time it is to say hello (in a formal way, like when speaking to strangers or professionally). [Unless maybe you're Quebecois, or from outside of France?]
When someone says "bonjour", 99.999% of the time it is to say hello (in a formal way, like when speaking to strangers or professionally). [Unless maybe you're Quebecois, or from outside of France?]