Markdown syntax fails to render inside custom tags

[hxj0x]

Bug Description

Markdown syntax fails to render inside custom tags when content spans multiple lines

Steps to Reproduce

import { Streamdown } from "streamdown";

// ❌ FAILS: "**bold**" renders as plain text, not bold.
// The content IS inside the component (unlike the header bug), but formatting is lost.
const markdown = "<ai-thinking>\n**bold**</ai-thinking>";

// ✅ WORKS: Single line bold works perfectly.
const markdownSingle = "<ai-thinking>**bold**</ai-thinking>";


function AiThinking(props) {
  return <div style={{ fontSize: 14, backgroundColor: "#3f3f3f3f" }}>{props.children}</div>;
}

export default function Demo() {
  return (
    <Streamdown
        allowedTags={{ "ai-thinking": [] }}
        components={{ "ai-thinking": AiThinking }}
    >
      {markdown}
    </Streamdown>
  );
}

Expected Behavior

When content inside a custom tag contains newlines, the parser should recursively process the inner content as standard Markdown.

Actual Behavior

Markdown syntax fails to render inside custom tags

Code Sample

Streamdown Version

2.5.0

React Version

18

Node.js Version

22

Browser(s)

No response

Operating System

None

Additional Context

No response