I've observed the following behaviour when painting terrain tiles with features:

Issue 1: Feature Persistence When Repainting
When painting over a tile that already has features, those features are not removed, even if you repaint the tile with a different terrain type.

Unedited Tile: Coastal[imgur.com]
Edited Desired Tile: Mountainous/Coastal Island/Increased Fish[imgur.com]
Repainted Tile: Coastal (features persist)[imgur.com]
Repainted Tile: Ocean (features still persist)[imgur.com]

Issue 2: Inconsistent Island Rendering on Coastal Tiles
An additional unexpected behavior occurs when painting a row of Coastal Island tiles. Some tiles fail to display the coastal node in the world view, while others show as flat in the preview. When launching a game, the in-game terrain matches the preview (i.e., flat instead of mountainous), and a visual glitch was also present.

Edited Tile: Preview Displays Flat/Mountain Tile Render[imgur.com]
Edited Tile: In Game Confirming Flat/Visual Glitching[imgur.com]

This mod is awesome! Hope this helps figure out what's happening. Let me know if I can provide anything in addition to this! < 3