The Diamond Rank has been added to Master Duel. Now you will need to be in Diamond Rank 1 to stop deranking as you did before in Platinum Rank 1.

Diamond Rank features another batch of rewards each time you rank up through its tiers, as the other ranks before it.

And additionally, the exploit some players used to disconnect right after the coin flip to avoid an unfavorable match and this act not counting as a loss has been addressed.