Facebook Hashtag Topics Do Not Filter Correctly

I have a private Facebook group that I use for dog rescue. Even when it looks like I have the correct hashtag topic assigned to a post, filtering does not always work. For instance, when I filter on #needsrescue, I see posts with the #rescued topic. I only use these two topics in the group. Each post originally has the #needsrescue topic assigned. I then remove this and add the #rescued topic after a dog has found a rescue. Some posts filter as expected when I make this change and others do not. I’ve tried adding a topic that did not have ‘rescue’ in the name and this has not worked either. Even when I remove all the topics from a post and save, I see the post when I filter for the #needsrescue topic.