Normal Terrain at Y:62 below Water level Y:63 It's not possible... if there is a cave on second photo more far. It's a terrain generation bug or normal terrain generation?

When I was searching place for my base I saw that weird generated place below Water level if Water level is on Y:63

seed: 6458361881194346504

Someone can explain, that cave would malfunctioned that generator of water level on first picture more far away...

If this bug or not, made by generator or malfunctioned generator of water, then explain