Alright so I finally found a way to make it work. My player was already in Fixed Update mode, so i tried to re-import the princess from your asset pack and set it up again from scratch.
When spawning my princess, I also had a clone spawning at the same time, which was due to the fact that my playerSpawn schematics was used as a start schematic + I had enabled the 'Start game' option on the Game starter in my scene.
I finally reused the princess prefab and replaced the sprites with my player sprites.
I'm really not sure how I fixed this to be honest but I'll be more careful with the game settings next time. Thank you very much for your assistance on this, I really appreciate it.