I'm in a similar spot, only difference is I got a 1-star review after a solid year of 5-star overall reviews. In short, here's what happened.
Recently, a guest showed up with his 6 yr old son, which exceeded my max guest limit AND in violation of my house rules that said my home is "unsuitable for 2-12 year olds." I was put on the spot, but given this was at night I let the family stay but told guest this put me in an uncomfortable situation. Guest cancelled remaining 2 nights online and the next day, wanted refund for those. I explained my no-refund policy and he got upset and repeated his request. After asking him more about why he brought his son, I realized this was a last-minute thing and offered to refund him one night. He remained upset and left. I still gave him the one night refund and since no good deed goes unpunished, now I have a 1-star review that's brought my rating down from a 4.89 to a 4.82.
Despite AirBnb's policy for guests to "not write biased or inauthentic reviews as a form of retaliation against a Host who enforces a policy or rule" this is obviously contradicted in the following response from their support team.
"In order for a review to be removed, the contents of the review has to be in violation of our policies, after checking several times through the review which was left for you, the review did not violate our polices. We also do not remove review based off truth or fairness."
"Based on the content in the review, we could not find any content or language that goes against our policies, we understand that this may seem unfair to you and wish that there was something else that could be done however, we are not able to make exceptions or override the policies."
So, it doesn't matter that the guest violated house rules, or that his review was untruthful, or that he admitted in his review that he violated house rules, only that his review didn't violate AirBnb policy....which is for guests to respect and follow house rules and not retaliate when a host tries to enforce them. In short, I think this is just about the most hypocritical policy ever seen from a world-wide business.