Hi Stefan Anton - I've done some research around this functionality and it looks like this has come up a couple of times in the past and at this time this has been deemed the intended behavior of the tagging functionality - Tag creation is not allowed in comments to Status Updates or Shares. Our engineering team has the following note around this piece of functionality we're seeing:
We do not allow tag create in comments because it would always result to a link to tag that doesn't exist in the system. Putting a hashtag in a comment box does not link that comment to the hashtag
It may be a good idea to head over to the Jive Platform team to raise this as an Idea for a future version - Our product managers review ideas in that place when prioritizing features for upcoming releases.