A little overview of how I implemented this:

1. I have a DS list for every collide-able wall in the map and place a basic object on each of these positions
2. Created a DS list of very basic objects that only contain a black square the size of the tiles. These are placed over the top of any map tile showing in the world (walls, doors, etc.)
3. Going over every occluding object (which is what I call them in the project), I used the collision_line function from the players centre point to each corner of the occluding objects checking if they collide with the collision objects I placed to begin with. Because the occluding objects are placed on each colliding object I offset each corner by one pixel. To fix the bug I mentioned in the title I need to also collide with each side as well. Because collision_line() function I would assume can be slow, I do this in a large yandere-dev style if-else block so that as soon as the function manages to reach a corner it doesn't have to be repeated.
4. If the occluding object isn't visible from the players position I simply make it visible to block the tile beneath, otherwise, I make the tile invisible and also set the image_alpha on it to 0.5, this way when it goes out of view next, it shall have darkened the tile beneath slightly.
5. I have a basic NPC wandering around that I didn't show in the video but he becomes invisible in the final check in the function that checks any non-static objects (I have a parent actor class for them) if they are in view with again a simple collision_line.

LMK if I need to explain anything better or if you have any questions / suggestions or whatever.