That is the achievement that apparently should unlock when all other achievements are unlocked.

Played the game twice. In the first game playthrough I unlocked all other achievments, but had to return to some cases to unlock them

In the 2nd playthrough I did ensure that I met all other achievements unlock conditions in one playthrough. Still the last achievement didn't unlock.

What gives?

Did you get or find an answer to that bug?

Having the same problem here. Already tried the workaround with Toby the dog, but it didn’t work for me (or I misunderstood what to do exactly...).

I have the same problem. Anyone managed to "fix" this?

Same !

March 2022, the problem is still there...

For anyone still stuck with this, read on!

The dog thing only works for those whose achievement that glitched was the dog one.
For me, it was the first case "Bruiser" achievement. I had an issue with my widescreen where the highlighted area was offset and so I could not win no matter what. I solved that by terminating the game and changing the resolution to make it work. I got the achievement but as it turns out it glitched and did not count toward perfectionist regardless.

I started a new game and played the first case and sure enough, after I won the fight I got the achievement. If you play the game from the start till you get to the achievement that glitched for you the Perfectionist achievement should pop.

If anyone needs further explanation go ahead and message me.