Previously, I ran the Ghosts of the Deep dungeon, solo on arc hunter. After the full clear, I got triumphs for completing the dungeon solo, with a full team of same class guardians and arc guardians. Cool, whatever. Later, my friend and I run it again, this time I'm on void titan, they are on void hunter. This is their first run of the dungeon. We run the first two encounters duo, try the final boss several times and take a break. They leave, I stay and walk around in the area for a bit. A bit later, they join back with another void hunter and we finish the boss. After all that, my friend has the Abyssal Void completed (for full void fireteam), but I don't - only the last encounter has been marked as completed. In case it's important - I didn't switch my subclass during the whole run, while my friend switched from arc to void at the very beginning.