Perry, I find out the bug of random hero script and the solution to it.
How to reproduce this bug?
Start a game with 8 players. Don't select random hero option from Diff Mod yet. Get to day 2 and enable random hero option from Diff Mod. Fight a neutral.

What exactly is wrong?
Enabling random hero option during the game on day 2 and after skips the important available player check on day 1 from random hero script. If there are 8 players existing, this script will disable itself but enabling it during the game breaks it.

And how did it fix?
I added player check after option being enabled by Diff Mod. So if there are 8 players existing, random hero will disabled itself as it should be.
Bonus: I also make the day 1 event to run daily. So now it checks whether there are 8 players everyday. If any player is kicked from the game and leaves an available colour, random hero will be re-enabled.

Download: Google Drive (updated 16 Oct)
Thanks to my mentor Hawaiing for his incredible help.