1. When placing an observer with its "powered" blockstate set to "true" via command, it will be placed with it set to "false". (Unlike MC-277277 the observer doesn't even detect a block update first, to turn off afterwards, but starts out as unpowered.)

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

2. Executing the same command a second time & thus replacing the unpowered observer (not just any block) will then make it stay powered as was expected from the first execution. (Every consecutive execution of the same command will then result in it failing as expected, because the block doesn't change anymore.)

• Execute the same command again