Hello!,
I can safely say now that the achievement bug has been officially resolved FINALLY!
The cause of this bug was found to be linked to the Skip button. When players would skip a conversation, it would completely break the games connection to the steam servers, preventing any achievements from unlocking further on. This explains why some players could unlock achievements without issues.
I apologize for this bug not being fixed sooner.
P.S
(I am also looking into the multiple endings not changing apparently & hope to rework it to work correctly)
Thanks!