Per Merriam-Webster.com:
: a person who takes part in
or becomes enthusiastic about something only when it is popular or fashionable
_______________
It would be nice if people stopped changing the meanings of words to fit their understanding. It is okay when individuals cannot agree about their favorite team. That is a disagreement.
It is not logical to label someone else a bandwagon fan if that other person continues having a conversation about their favorite team. By definition, a bandwagoner would stop talking and move on to something else that interested them more. It is possible they might re-claim fanship for a team they stopped following at a later time but that action would further validate the term's actual definition.