The diamond ore generation was fixed in 1.17.10.21 beta (relates to MCPE-127555), however, if you run the world with the base game version set to 1.17.0, it will still generate with the wrong distribution. This ticket would most likely affect marketplace and world template worlds.

You can look at it in the game files and go to vanilla_1.17.0 diamond ore features JSON file in the behavior pack folder to check if it's fixed. The count should be "8", instead of "4", if you compare with 1.17.10 diamond ore features JSON file.