Okay so I have played a bunch of mannequin horror games and I have to say that this is one of the best easily. It's such a simple and short game and with a simple mechanic but makes good use of it and in a good way. (Video below for anyone interested!)
So the story itself I found intriguing and engaging throughout the game; even after getting captured, you get to see and hear yourself as a mannequin, which I think is a great touch that you guys added and not many other mannequin horror games have.
The sounds when you were running away from the note; the sounds of the mannequin where actually terrifying (for me at least) though I found it strange that there as a metal clang noise for the first two flashes, but the third one was spot on and worked really well with the mannequin suddenly teleportation behind the player to make an effective scare.
The atmosphere I think you nailed as well; just a simple room and you get to see the mannequins and anticipate what happen before hand; which serves as a good climb to the climax.
There are only three things that I found this game a little lacking were the story and explanation; I wish there was more of an explanation as to why light stops the mannequin from moving. (And also I only just realised that it was the light in the room that were keeping the mannequin at bay and the reason they move was because the light went out and hence they are repelled from the flashes.) But an explanation in the future might be good.
The other suggestion I have is to reduce the length it takes for you to die and then go to the playable part of the game again. I found that it was faster to force quit the game and go to the playable point again rather than go through the whole death scene (I died a few times before I figured out what I had to do and hence came across this.) So maybe adding a skip scene button might help with this.
The last thing would be adding a death animation of the mannequins actually attacking you. The sounds are scary as it is, so adding an animation where the mannequins jump scare you/ attack you would make the death that much scarier and would incentivise the player to try to not die. (Also the metallic clang makes the cue for the flashes a bit too easy, so I suggest just keeping the scary and silent shuffle, which I think is way scarier than the former sound.)
But all in all, it was really fun to play. I did not expect a game that was that short to be that good and was really surprised as to how scary it was. Good job making this game, and good luck on future updates and projects!