To reproduce:
1. Set JVM arguments in Profile Editor to:
-Xmx512M -d64

2. “Play”
3. Receive “Client> Invalid maximum heap size: -Xmx512M -d64” error

P.S. Just in case, initially I’ve tried to launch the game with these args:
-Xmx512M -d64 -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:+AggressiveOpts