How to reproduce:

1. /give @s minecraft:command_block 

2. Put in the command block the code:

3. setblock ~ ~ ~2 oak_wall_sign[facing=north]{front_text:{messages:['{"text":""}','{"text":"Example","color":"red","bold":true}','{"text":""}','{"text":""}']}} keep 

4. Set the command block in repeating mode always active
5. Try to break the sign 
6. The text of the sign won't show it but using: /data get block <coordinates> you can check that the nbt data of the text has to be showed
7. In fact when exiting and re-entering the world the sign will show the text correctly

Expected Result:

The sign even if destroyed and replaced should show the text

Onedrive video:

BugMinecraft.mp4