Jive has told us the issue is solved. Once they push the next update to the site it should be fixed. Regrettably we don't have a date on when that will be. You can right click on anyone's name who has an underscore and copy and paste the link to their profile into the field and it will resolve to an @mention of them. I apologize for the inconvenience this has caused. Because there is a fix in the future there aren't any plans at this time to prevent new people from creating a username with an underscore. Furthermore with there being a couple workarounds it's isn't as high on the priority list compared to the other things that are being worked on. I realize it is frustrating. Had we known that it was an issue we definitely would have made some different choices before we updated usernames prior to the move.
If you would like your username updated to remove the underscores, I would be happy to do so but I'll handle it on a case by case scenario.
Update 11/1/2017: Realistically this will not be fixed until Q1 2018 now. If you would like your username updated to use hyphens or shorten your username that will work too.
Update 1/30/2018: Realistically this will not be fixed until Early Q2 2018 now.
Update 10/26/2018: This was fixed today.
Mark
EX Community Manager
EX Community Admin Team
