1. When an observer is placed using a command, it will "observe" a block update and thus switch its "powered" blockstate to "true" for a brief moment. This is unexpected because placing an observer by hand doesn't cause such an activation.

• /setblock ~ ~ ~2 minecraft:observer
• /fill ~ ~ ~2 ~ ~ ~2 minecraft:observer

2. The same occurs when the command fails to place the observer when there's already an observer placed at the target position (see the attached screenshot).

• Execute the command from above again (i.e. two or more times in total)

3. The behaviour from both 1 and 2 even occurs when the blockstate "powered" is specified in the command as "false".

• /setblock ~ ~ ~2 minecraft:observer[powered=false]
• /fill ~ ~ ~2 ~ ~ ~2 minecraft:observer[powered=false]