Reproduce

1. Place a sign (doesn't matter if you type something)
2.

/data modify block <pos> front_text.messages[0] set value {text: "test", bold: true}

Expected

The sign has the text "test" in bold on the first line.

Observed

Error message "Nothing changed. The specified properties already have these values".

More info

This is the only way to do the same thing, but it will override any existing text:

/data modify block -262 64 2 front_text.messages set value [{text: "test", bold: true},{text:""},{text:""},{text:""}]

A more complicated setup with a macro that has arguments for the existing text could in theory be used.