I’ve just had a quick repro attempt on my test site but the new tag + description onebox is working for me. But… I have been lazy and not updated for awhile (1a5377cf93).
Let me update and try again, and then if it doesn’t work on a second test then we can at least rifle through the intervening commits and maybe narrow down if something has happened in the interim. 
After updating (8f89e287d3) and attempting again (both with an existing tag + new description as well as a new tag + description) it all seems to be working fine. The tag link oneboxes on another site with the tag description included.
@Damian_Boon Is this happening for all of your new tag descriptions or only some?