So there are many blocks that stop water source blocks from spreading/water from flowing. Water source blocks stopped by these types of blocks usually have animation of flowing down, but buttons make it behave like its touching a solid block and there is no flowing animation.
To check it out by yourself:
1. Make a column of water source blocks
2. Surround it by something like fences or anything else that is not a full block and stops water from flowing
3. Replace one fence or whatever you used with a button (place a block to place it on if needed)
Water should have animation of flowing down where it's adjacent to the fences, but static where it's adjacent to the button

Example is on attached screenshot