As a beginner, you have no expectations. You do not know what’s possible. What can you win, what can you lose. There is no pressure of playing which is why you are just able to win. But a few rounds after that, you are already expecting to win. There is the need to perform better and get more money than the previous round. And that is the explanation for a beginner’s luck. It’s just our mindset.
I agree with your conclusion still how more than 50% of beginners (in both trading and gambling) are able to see profits (please note only 'see') whereas non-beginners face loss even on the first round itself. For example, if now I open and play dicing, my first roll will be against myself and then I need to go martingale and then ned to reset the seeds so that I could recover what I lost in my first roll.
I know someone that gambled for the first time that lost the game immediately and he told me he is no more gambling again.
Unfortunately, my ever first dicing was profitable and I still remember it. I was about plan to collect up to 1 BTC from the faucet claiming but suddenly faced consequent losses and martingale led to zero balance. I learned martingale strategy before that from this forum and I googled how to profit from dice gambling using that strategy then tried on a crypto gambling house still I was able to see my profits but never got realized.