In 2020 Yammer separated Topics and Tags as separate items. Before when you hash-tagged something, Yammer assigned that tag as a topic to the thread. Under the new Topics experience you can use a hashtag (tag) inside a conversation and Yammer will not assign it as a Topic in the thread.
See this article from Microsoft for Details.
tyGraph Topics & Tags
At tyGraph we are capturing both topics and tags but are counting tags under the thread as though they were a topic. In the future we may add topic and tag selection as a report option. Currently both are lumped under topics.