Give us the steps to reproduce the bug

Gemstone Caverns and multiple leylines are in your opening hand.
You select Gemstone Caverns and one leyline to put on the battlefield.
You wanted to exile another leyline by the Gemstone Caverns.

  Note which platform you’re using (PC, Steam, Mac, Android, or iOS)

PC

  Enter the actual results you’re experiencing

Gemstone Caverns cannot distinguish which leyline is the one onto the battlefield.

  Enter the expected results of the feature

Plan A : Gemstone Caverns can distinguish which leyline is the one onto the battlefield.
Plan B : The processing of Gemstone Caverns takes place after leyline.

  Add any relevant screenshots/videos

None

  If you found a workaround, please add workaround steps.

Probably full control is workaround.
However, that is a bit unkind.