I can't really speculate on that topic. It's currently an active area of research.

To be honest, this problem is so widely known that I hadn't considered finding sources to support the claim. Here is the best authoritative source I could quickly find

https://arxiv.org/abs/2012.15613

It may seem counter-intuitive to link to a paper that supposedly fixes this issue, but this is obviously the most likely scenario in which a paper would discuss it. Also, if you read it carefully, you'll see that while the authors managed to reduce the gap, it still persists.