The letter theta "θ" in RTF text is displayed as "?" in RichTextBox

[GowthamabalajiSambathSF4978]

Describe the bug

We are facing an issue where the letter theta "θ" is displayed as "?" in RTF text when using the MS RichTextBox control. When attempting to display special characters (e.g., λ, θ), some of them are not shown correctly and instead appear as "?".

However, certain special symbols are rendered correctly, while others are not.

For example, when using the code snippet below in the button click event with the Beta letter "β", it works properly.

Code snippet:

private void button1_Click(object sender, EventArgs e)
{
    string rtf = "{\\rtf1\\ansi\\deff0" +
               "{\\fonttbl{\\f0\\fmodern Arial;}}" +
               "{\\colortbl;\\red0\\green0\\blue0;\\red230\\green230\\blue230;}" +
               "\\fs22\\f0\\cb2\\cf1 " + "β" +
               "\\cb0\\par" + "}";

    richTextBox1.Rtf = rtf;
}

When using the code snippet below in the button click event with the theta letter "θ", it does not work properly.

Code snippet:

private void button1_Click(object sender, EventArgs e)
{
    string rtf = "{\\rtf1\\ansi\\deff0" +
               "{\\fonttbl{\\f0\\fmodern Arial;}}" +
               "{\\colortbl;\\red0\\green0\\blue0;\\red230\\green230\\blue230;}" +
               "\\fs22\\f0\\cb2\\cf1 " + "θ" +
               "\\cb0\\par" + "}";

    richTextBox1.Rtf = rtf;
}

Sample Zip: WindowsFormsApp1.zip

Steps to reproduce the bug

1. Run the sample application.
2. Enter the theta "θ" character in the RichTextBox.
3. Click the "button1".

Expected behavior

The letter theta "θ" should be printed correctly.

Actual behavior

The letter theta "θ" not printed properly.

Screenshots

RichTextBox-ThetaIssue.mp4

Windows version

No response.

Additional context

No response.