I found this "solution", and I'm using quotes because I don't know if this really is a solution or not, not to mention that it is risky to do. Feel free to correct me on this.

1. Go to Documents\My Games\Terraria\tModLoader\Worlds or any place that your world files are stored.
2. Find a folder that has the name of the world where you are having issues with
3. You will see files with the word "MothWorld" in the name. Delete them.
4. Go back to the worlds folder and find the file with the world where the subworld issues occurred. As long as you don't have any important buildings or anything on there, delete the file, and all other related files (world.wld, world.twld, world.bak). This is the same process as deleting the world in-game.
5. Make a new world and enter it
6. If entering the Firefly Subworld doesn't work, exit the world and close tModLoader, then start tModLoader again.
7. If it still doesn't work, delete the world, close the game, open the game, make a new world, and you should be good to go.
8. If it STILL doesn't work, repeat step 7, and you will need to enter the world using a new character. If it works, try with your older character that you would have used. If it works with your older character, then you should be okay.

9. If nothing works, Disable Everglow, delete the world that the subworld issue was present in, delete any saved subworlds that are part of the issue from step 1-3, delete any characters that were used for entering the subworld (if needed), and then close the game, reopen the game, enable Everglow, make a new character and world, and then see if you can access the subworld with very little issues.

If all else fails, disable any other mod that heavily utilizes subworlds, and repeat the steps until you are able to enter the subworld and see the tiles and everything else correctly.

And finally, if you want to leave the subworld, go to the in-game settings and click "return". Do NOT enter the Moth Door again unless you are in the overworld.