self explanatory. Once the base block for one of the crystals is broken, a super massive lag spike happens (no crash though, just jagged movements) until the floating crystal is broken. after that, game proceeds to work as if nothing ever happened.

Forge: 1.12.2 (*.2515)
Mod: 1.12.1-r121

seems inconsistent, will reopen if it becomes consistent, seems to be an mod interaction thing