I can confirm what @alehandrof describes on my site, though I don’t see it as a high priority. However, I agree it’s a bug if the intention is to give full citizenship to groups.
Mentioning groups seems to require you to start with the first letters of the first word of the group username. See screenshots below.
Mentioning part of full name of users - works:
Mentioning first letters of first word of group username - works:
Mentioning part of group username that appears later in the name - does not work at all. Nothing happens.
