EDIT: this got automatically fixed for me after I logged in after 6.1 got deployed.

I was actually at 31/35 in the ordeal overview and still had 4 unfound dataslates in the repository.

After I found the 4 missing dataslates it seems that one of them did not register for the ordeal. I did get the achievement but not the ordeal (which is now stuck at 34/35).

I did find all 4 missing dataslates while playing offline. It doesn't seem to be necessary to find them online but maybe it caused one of them to not being tracked.

One of the four missing dataslates was #2 on Dawns' Descent. I also did the "hit-10-objects ordeal" - which did not get registered as well.

I didn't really track the ordeal until I found all missing dataslates but maybe the one not being registered was the one from this mission since the "hit-10-objects ordeal" did not register as well.

I could just redo the "hit-10-objects ordeal" afterwards (online) but was not able to pick up the dataslate again.