I do not know how to fix this.
So no PR from me.

The core block var do not work ( like "{group.user.USERNAME_FULL}" ) in event template,
Works very well in "memberlist_team.html" but not
in "/ext/{Vendor}/{EXT}/styles/all/template/event/memberlist_team_username_prepend.html"
or "/ext/{Vendor}/{EXT}/styles/all/template/event/memberlist_team_username_append.html"

Template Event HTML File :

OutPut :

If any more details are required , then kindly let me know.

Hope to resolve this bug soon.

Best regards