The 10 (and possibly 20/30) point traits you select become empty trait slots when going from the pve environment to the Heart of the Mist. This only happens if you have trait in a shared line between modes; such as having 10 points in Arms in pve, and 30 points in Arms in pvp, results in an empty 10 point trait slot in the pvp hero panel when you go from pve to pvp.
I didn’t have this issue until I spent trait points in pve and unlocked my first 10 point trait.