Title+videos are pretty self explanatory.

EDIT: environment information:
Minecraft 1.16.5, singleplayer
mods.txt

Please provide some basic information, like which MC version are you on? Is this on a server? And what other mods do you have? Because this does not happen for me in my single player test instance, nor on a local test server.

Sorry about that, added info above.

That's a lot of other mods... I'm not going to spend hours testing all of those. See if you can narrow it down by bisecting the installed mods (disable half at a time).

And actually you can probably narrow it down first by adding the -Dmixin.debug.verbose=true option in your JVM arguments in the launcher, and then launch the game with that once, and then in the console output search for all lines that have either class_498 or SignEditScreen (I'm not sure if Not Enough Crashes is going to deobfuscate that output, probably not). That will tell you if there are any other mods that modify the sign edit gui.

I can't see how Tweakeroo's closing the gui alone would cause it to not cause block updates. Is the sign block actually placed (on the server side)? If you re-log, is the sign there?

Ok I've narrowed it down.
With just

• tweakeroo-fabric-1.16.4-0.10.0-dev.20210303.123654
• malilib-fabric-1.16.4-0.10.0-dev.21+arne.2
• fabric-carpet-1.16.5-1.4.24+v210120

I get this issue in this save: New World (4).zip

If I re-create the world, the issue doesn't occur, and if I copy over the world's carpet.conf to a different world, the issue doesn't occur.

Very strange

It does not happen to me in that world with those mods. When loading the world I noticed that Carpet fails to read a rule called editableSign, which sounds like it might be related, and must be from some addon mod?

editableSign is from Ivan Carpet Addition, but it wasn't installed, it was just the three mods above...