When using setblock to place a pressed button or replacing a pressed button with a non-pressed button, redstone connected to the block the button is placed on is not affected and the redstone will only update once a block update is provided.
To recreate this bug:
1. Place a command block with command: setblock ~-1 ~ ~ minecraft:stone_button 2
2. Place a stone button on the west facing side of the command block
3. Place a piece of redstone next to the command block
4. Press the button
When the button is pressed, the command block replaces it with an unpressed button, but the redstone stays on.
This works also for wooden buttons, but not if the button and the button in the setblock command are different.