This is how to replicate this bug.
Step 1: Log in to your dueling book account and click "DUEL!"
Step 2: Scroll down to where it reads "Custom Cards".
Step 3: Click "Custom Cards" and you will be taken to the custom card editor screen that looks like this:
Step 4: Create any custom monster that is either a Sea Serpent or Winged Beast monster.
Step 5: Click the "Upload" button at the bottom right corner of the screen to add it to your custom cards.
Step 6: Click on "My Cards" located at the bottom left corner of the screen, and hover your mouse over your newly created custom monster.
Step 7: Click on your newly created custom monster.
As you can see, after you bring up your newly created custom monster, you will see that it's "Type," "Ability," and "Attribute" are all blank, even though you specifically made it a Sea Serpent or Winged Beast monster. If you want to edit this custom card, you will have to manually give it a Type and Attribute again if you wish to save it ("Abitlity" is optional).
For some reason, this bug never occurs with monsters of any other Type. It only occurs with Sea Serpent and Winged Beast monsters. My guess is that it has to do with the fact that these types have spaces in between them, and that somehow messes the monster up.
Another bug involving custom Sea Serpent and Winged Beast monsters is that if you select a different custom monster of yours, and then go back to and click on your newly created Sea Serpent or Winged Beast custom monster, you will find that the Level, ATK, and DEF of the other monster transfer over to your newly created Sea Serpent or Winged Beast custom monster that you clicked on. For example, when I clicked on one of my other custom synchro monsters which is a level 8 monster with 3000 ATK and 2500 DEF...
And then I go back to my newly created custom Sea Serpent/Winged Beast custom monster and click on that...
You can see that the level, ATK, and DEF somehow transfer over to the newly created custom monster. At first it was a level 1 with 0 ATK and DEF, and now it becomes a level 8 with 3000 ATK and DEF, simply because I clicked on my other custom synchro monster with those stats.
This bug has existed for several months now and I am surprised it has not been addressed yet. If it could be addressed, that would be appreciated, since, right now, creating any custom Sea Serpent or Winged Beast monsters, or any kind of mermail, atlantean, blackwing, or simorgh support that is one of these types, is a pain in the but.
