I'm so sorry, I just found that there was indeed a bug with the keys. The blue key accidently had the same ID as the other key, so when you collected it and saved, the game thought the other key was also already collected, so it didn't respawn when loading the game again.
Thanks so much for helping me find this bug, and sorry for the inconvenience :)
Please download the new version and the issue should be fixed (the key should now actually be at the spot B4).