Due to text components being changed from strings containing JSON to NBT structures the messages lists in signs can be either a list of 4 strings or a list of 4 compounds. If all lines are unformatted, they resolve to strings.

Due to this, it is no longer possible to replace a line with formatted text if the line current is all unformatted, because the list cannot contain a compound and 3 strings.

This is a breaking change as before it was always 4 strings regardless of text formatting.

Steps to Reproduce:

1. Open a singleplayer world in 1.21.4
2. Place a sign
3. Stand inside of it and run

   /data modify block ~ ~ ~ front_text.messages[0] set value '{"text":"Hello","bold":true}'

4. Observe the sign
5. Close your game and relaunch in 25w02a then reopen the world.
6. Destroy the sign and replace it
7. Stand inside of it and run

   /data modify block ~ ~ ~ front_text.messages[0] set value {text:"Hello",bold:true}

8. Observe the sign

Observed Results:

In 1.21.4, the bold line correctly replaces the first line. This works because you are changing the list from

["","","",""]

to

['{"bold":true,"text":"Hello"}',"","",""]

In 25w02a, the command that you would expect to do the same thing fails because you are essentially attempting to change

["","","",""]

to

[{bold:true,text:"Hello"},"","",""]

which is illegal as NBT lists may only contain a single type.

Expected Results:

In both versions, the respective command should modify the first line of the sign to say "Hello" in bold.