That’s because you need to select…
- 15 5-star gems
- 16 2-star gems
- 16 1-star gems
I explained this, with screenshots, here…
https://us.forums.blizzard.com/en/diablo-immortal/t/select-legendary-gem-pool/6653/2
Notice the tabs on the left of the selection screen…
5-stars…
2-stars…
1-stars…
This is NOT a bug.