Mosstrosity secret boss not unlocking

I have unlocked all the talismans throughout the game, including the secret ones; however, for the last three of the secret talismans, it did not show the pop-up that I discovered them, and I did not receive the prompt that the secret area in the overworld was unlocked. I have completed the main story and the game. I am collecting the area items now; I just want to find the secret boss so I can fight it and see what it looks like. I have restarted my game. I have manually ended the running files through task manager for it, but nothing works.