Drowned mobs spawning in water 1x1 water source blocks, even spawning in with transparent blocks above their heads

What I expected to happen was...: No mobs should spawn
**

What actually happened was...: Mobs spawned

Steps to Reproduce:
1. Create a 1x water stream using ice and surround the top and sides of the stream with glass and place a water source block down. Drowned mobs will spawn in.