The observer seems to update air or any other block behind it (even other observers which are marked as a transparent block) with a redstone signal. It can be seen with just a normal block update but I found it by running into MC-107500.
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Maybe the observer is sending out redstone updates even if no redstone to update is present.

Confirmed in 16w39b

Works as intended - just like the repeaters, the observer block will update adjecent blocks, when powering the block at it's face. This applies even if there is no block in front of it. Since the observer block checks for updates, it will power, when this happens.
Making a loop of observerblocks however is something different, and does not seem to work correctly.

@unknown that goes against the expected behaviour, as you don't see/force a update yourself.
We're leaving it open for mojang, there's no statement anywhere from them that this is intended.

In that case, many things would go against expected behaviour. I'll leave it to Jeb to decide, but seeing no legitament "fix" for this, as it is how redstone works, there wouldn't be a point in looking into it. I'm simply trying to save time on mojangs part, and make them fix bugs instead. But as you say, it's up to mojang to decide.

The thing is, the block doesn't actually update in any way, so the observer should, logically, not output.

Observers activate on a block update. Observer blocks in this case will update any block with a Manhatten distance (search it up, if you're confused) of 2. 😉
However.. The problem here should be changed. Yes, it is stange, that the observer blocks stay activated, and I see no real reason for this.

The observer blocks are meant to work as your standard block update detectors using budded pistons. The standard block update detector detects these invisible redstone updates.

Probably no one cares about this ticket anymore but i'll just say this: this was not a bug! This is how piston BUDs always worked because redstone components that can power blocks ALWAYS give out updates at a range of 2 in the direction they are able to power. This "fix" needs to be reversed.