In Customized Superflat world generation, Diamonds oddly generate way higher than they should. While they normally stop at y16, in my testing, diamond veins reached up to y73.

For some reason, no other ores bypassed their orthodox limits.

(in the steps below, you could have the stone go up to the world limit, but y64 is recommended, as it will lag much more and you'll have to go much deeper to find what you're looking for.)  

Steps to reproduce:

1. Customize the Overworld preset to have 63 deepslate between bedrock (on bottom) and stone (on top), so ores generate normally.
2. Execute the command /gamerule commandmodificationblocklimit 1000000 (or some other big number)
3. Using /fill, replace all blocks except for ores using "/fill <pos> <pos> air replace (stone, gravel, dirt, andesite, diorite, granite, lava, grass_block)
4. Notice how diamonds do not stop generating at y15 or y16