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Actually, Oskar's response seems to be more correct because if you place a torch on a redstone block, it will unpower and if you place a redstone on the output side of an observer it will turn on.

Anyhow, this is working as intended.

As far as the five images are concerned, we can't take any conclusions if we don't know how the piston was powered. As far as the last image is concerned (pistons extended with redstone blocks on top), that is a know feature, called Quasi-Connectivity.

You are missing a } at the end in the command.
Correct command would be:

/data merge block 20 65 -2012 {Text1:"[\"\",{\"text\":\"Population of \",\"color\":\"black\"}]",Text2:"[\"\",{\"text\":\"Felidale\",\"color\":\"black\"}]",Text3:"[\"\",{\"score\":{\"name\":\"fakePlayerName\",\"objective\":\"c_count_total\"},\"color\":\"black\"}]"}

Sorry to be bringing this back up but I really think this should be fixed and that this is a serious bug, even though the developer team might not want to face it as one.

So, what is my reasoning?

summon minecraft:zombie x y z {NoAI:1b}
execute positioned x y z run data modify entity @e[type=minecraft:zombie,distance=..2] CustomName set value [{\"nbt\":\"Health\",\"entity\":\"@e[type=minecraft:zombie,distance=..2]\"}]
data modify entity @e[type=minecraft:zombie,distance=..2] CustomNameVisible set value 1b

summon minecraft:zombie x y z {NoAI:1b} 
setblock x y-1 z minecraft:oak_sign replace
execute positioned x y z data modify block ~ ~-1 ~ Text1 set value "[{\"nbt\":\"Health\",\"entity\":\"@e[type=minecraft:zombie,distance=..2]\"}]"
data modify entity @e[type=minecraft:zombie,distance=..2] CustomName set value ""
data modify entity @e[type=minecraft:zombie,distance=..2] CustomName set from block ~ ~1 ~ Text1
data modify entity @e[type=minecraft:zombie,distance=..2] CustomNameVisible set value 1b

So, what's up with this? The first one doesn't work, for reasons this bug described. The entity cannot process selectors in JSON Compounds and that is horrible. The fact that it doesn't give us that freedom makes it so that we have to work around it using the 2nd set of commands, which is horrible for performance, as it sends packets to every client connected to the server and each client then needs to update what the sign says (and the server also needs to do that). Long story short, the fact that selectors are processed in entities' nbt limits our possibilities and forces us to search for over-complicated solutions to simple (but essential) problems (like setting the name of the entity to its health), and hurting the server's performance and the players' experience in the process.

In case you haven't checked out the "duplicate" bug report I made, here is a world showing some new unwanted behaviour of repeaters introduced in this pre-release (Bug World.zip)

I beg your pardon, but, in my opinion, even though, those two are very related, neither of them is a duplicate of the other...

The issue described is not a bug and is working as intended. It is called quasi-connectivity and you can learn more about it here.

Someone correct me if I'm wrong, please.

In 1.14, this only appears to happen when entities are going from the nether to the overworld, not the other way around...

After further testing, I have verified that the issue was being caused by a datapack I had installed in my world. Sorry for the inconvenience!