Thanks a lot for your honest feedback, it is very appreciated!
This game prototype was made very quickly, and we focused almost exclusively on the underlying concept. The execution is indeed poor, also because of our inexperience.
We are currently working to reach a more advanced platform code, that would hopefully be used to fix the issue you experienced.
About the diamonds: our intention was to provide a simple goal for the prototype, i.e. to collect all diamonds. Diamonds do not reappear when you respawn because you keep the ones already collected. The goal should be reached throughout many generations. The idea was to make a fast-paced platformer in which you'd have to collect the "diamonds" through your generations while the life will pass relentlessly. You would need to collect the diamonds just once, then you can go fetch the others if your lifespan is over or if you die by accident :D
For the build version, I am sorry it didn't work on your system, may I know which OS you're running ? I just tried with my macOS and it worked.
Hope you will give it another try if we manage to transform it into a real game! Thanks again.