Hello and congratulations on your release of v1.13.1 for Bedrock Edition. The light block is brilliant, no pun intended. The bug being reported is an ongoing issue from 1.13:

Edit: further testing shows that this bug only occurs when glass blocks are used above ground

Steps to recreate this bug:

1. Build a vertical water column with glass blocks above ground (edited 2113 11/12/19). (bug affects both up or down)
2. Verify each water block is source block to ensure proper column function
3. Create an entrance using signs (to stop the water flow)
4. The exposed water will adopt a dark reverse pumpkin pattern texture

This is not a texture pack issue as I recreated this using default vanilla textures. Nor is it related to any user adjustable video settings in game.

Included is a screenshot of this issue.