Date & Time: May 26th 2018, 13:11PM
Frequency: First time for myself, but have seen multiple reports of the same issue.
World name: Phoenix
Character name: Djessica Taimi
Retainer name: None
NPC name: None
Monster name: Nybeth Obdilord
Class/Level: Scholar 60
Party or solo: Solo
In-game time: Not recorded
Area and coordinates: Palace of the Dead, floor 100
Housing: None
Steps:
  1. Enter Palace of the Dead solo with existing save on floor 91. No prior deaths have been recorded on this save.
  2. Clear floors 91 until floor 99 and reach floor 100.
  3. Engage Nybeth Obdilord and complete the duty.
  4. Instead of earning the achievement Pal-less Palace II, earn Pal-less Palace I instead.
    note: When clearing floors 1 through 50 without failing the duty, I never earned the achievement Pal-less Palace I. Most likely that isn't working properly either.
  5. Aware of this issue possibily happening, the advise was to record a score on floor 101 or higher.
  6. Enter Palace of the Dead solo with recently updated save on floor 101.
  7. Finish floor 101 and proceed to floor 102.
  8. Trigger a landmine on floor 102 and be defeated by an enemy.
  9. A score is recorded on floor 102 but no achievement is earned.

Below is a small transcript of the events and the timeframe of the above steps.

[12:30]The Palace of the Dead (Floors 91-100) has begun.
...
[13:07]Floor 100
...
[13:11]You defeat Nybeth Obdilord.
[13:11]You earn the achievement “Pal-less Palace I”!
[13:13]The Palace of the Dead (Floors 101-110) has begun.
[13:13]Floor 101
...
[13:16]Transference initiated!
[13:16]Floor 102
[13:17]The landmine is triggered...
[13:17]You are defeated by the deep palace sprite.
[13:17]The Palace of the Dead (Floors 101-110) has ended.

The achievement descriptions are:

Pal-less Palace I
- Clear the 1st through the 50th floors of the Palace of the Dead
solo and record a score on floor 51 or below.

Pal-less Palace II
- Clear the 1st through the 100th floors of the Palace of the Dead
solo and record a score on floor 101 or below.

In this scenario, I expected to earn Pal-less Palace II and not Pal-less Palace I, as the criteria had been met.
Not earing the Pal-less Palace II achievement basically forces me to run the entire thing from floor 1 to 100 again, which is absurd.
Either the wording in the achievement is wrong or the achievements still aren't being flagged correctly as completed.