Stypox

Now it seems to be fixed.

@Conrad Herman

I used two "/fill"s for every possible orientation of the observer (north, south, west, east, up, down).

The first one replaces every observer facing in a certain direction (e.g. North) with a dummy block (e.g. Bedrock, that is not part of the volume selected).
The second one replaces the dummy block (in this case Bedrock) with the observer in the chosen direction (in this case North).
Example:
1) /fill x y z x1 y1 z1 bedrock 0 replace observer facing=north
2) /fill x y z x1 y1 z1 observer facing=north replace bedrock

You have to do the same thing for every direction (6 times).
Remember to use "strange" dummy blocks (like structure_void) to be sure not to replace other blocks.
Works in 1.12.

A thing I noticed while using MCEdit is that the stuck-buggy observer becomes a non-recognized block: this means that it has invalid tags.
I think it doesn't get updated because of that.

It happens on servers too. On a big survival multiplayer whitelisted server all the observers got stuck and we had to replace them with "/fill"
P.S. The server was 1.12 vanilla Minecraft (without plugins or mods).

I can confirm this issue

✔ I attached the crash log file. I hope that's the right one. I use Minecraft on italian language, so sorry about it...

If the game is crashing are you able to attach the crash log file to the issue?

@Derrick Lambert