Summary
I was working on a Survival world where I wanted to raise the water level of a lake and river, so I built a platform (cobble) below the desired level and placed water until infinite across the area, finally removing the cobble platform to release the water.
The issue is the grass block will not update to dirt, even after 20 minutes no updates. Mobs that were under the platform behaved as if they were on land after I broke the cobble and did not drown nor catch fire.
Expectation
Raise water level of a river/lake several blocks (from Y=62 to Y=68)
Reality
Expectation was met but with three minor bug discovered.
Previously existing grass blocks will not decay to dirt blocks.
Monster Mobs spawned (as expected) under the platform, but as water fell after breaking the platform, they behaved as if they were on land (water slowness in affect) and not drowning nor catching fire in the daylight. The zombie shown in the screenshot eventually turned to a Drowned and swam to the surface to attack.
Cobblestone blocks had a equal chance of sinking or rising when broken.
Steps for Replication
Create a platform 4 or more blocks high from the surface of grass (I did mine over an existing river and lake in a survival world)
Place water buckets on top of the platform until all blocks are water sources and still.
Place or let mobs appear under the platform
Destroy the platform
Results and Data
Skeletons don't swim in water, but walk on the ground and act as normal. Same for zombies.
Grass blocks only decay to dirt blocks if they have a solid block directly above them
You created a lot of flowing water. Flowing water will push items down.
Everything you described sounds like normal behaviour to me.