Canidate Trials
Have you tried doing t2 again. Just a thought but it might help.
I had the same issue. I went through and did t1 and t2 again, but only t2 gave me credit for the second half of the achievement. So doing just t2 again may work.
Can confirm, did t2 (ding), t1 (nothing) and t2 again (ding)
80’s: Engineer/Warrior/Necromancer/Mesmer/Thief/Elementalist/Guardian [Seafarer’s Rest]