I quickly dropped it all and inspected all of my arrows. Sure enough I found another few with the same issue and other that were on the way as well...As you can see - the plastic bottle cap is already fractured. Obviously my stress testing which was indeed a bit rough, showed that the Hungarian recurve was too strong for ordinary boffer arrows.
Therefore I went for making better ones, before Someone gets an arrow in the eye...or knee.
For this, I decided on two things - adding a cork, and a coin to the head. That's a simple wine cork, leftover from New Year's eve, rest is pretty clear.
Then the foam layers are added. One side layer, 2 top layers to be precise.
And its done. I will put it to the test soon enough. Making more will be a bit of a problem, since I don't have that many corks around. I might have to find a replacement for it.
Moral of the story? Stress test and double check your arrows!