This is not a server, but a clientside rendering flaw. I've been able to reproduce this issue on vanilla single-player worlds across multiple machines, are you running the commands in rapid succession, and are you specifying a user instead of just "victim"? I have to run the commands about seven times over in order for it to crash. I haven't been able to reproduce the issue in the 1.16 snapshot, but it should be reproducible in 1.15.2.
This is not a server, but a clientside rendering flaw. I've been able to reproduce this issue on vanilla single-player worlds across multiple machines, are you running the commands in rapid succession, and are you specifying a user instead of just "victim"? I have to run the commands about seven times over in order for it to crash. I haven't been able to reproduce the issue in the 1.16 snapshot, but it should be reproducible in 1.15.2.