When sending a signal through a honey block with an observer output, the signal travels downwards but not upwards.
To reproduce:
1. Place honey block at observer output
2. Place pistons on top and bottom of honey block
3. Activate observer with some block update
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Honeyblocks are not full blocks and thus cannot transfer power. That the Piston diagonal below the Observer still receives power is MC-108, "Quasi-Connectivity", which is a bug that is since then tolerated, as its removal would upset some people.

I don't know how to reply to your comment but does Quasi-Connectivity not work in all directions then?

@unknown No, Quasi-Connectivity (QC) doesn't work in all directions. You can read about it here: https://minecraft.gamepedia.com/Tutorials/Quasi-connectivity.
Keep in mind that the Redstone system can always change, and that it's - at least currently - different on Bedrock/PE (no QC).
Take care!