[Python] RTF Output Shows "{ Apple0}" Instead of Bold "Apple" in LibreOffice on Ubuntu

I'm writing a Python script to generate an RTF file containing text excerpts with a search term highlighted in bold. The script successfully generates the excerpts with the keyword, but the term isn’t bolded as intended.

Search Term: "Apple"

Expected Output in LibreOffice: Apple (bolded)

Actual Output in LibreOffice: "Apple0" (plain text with "0" appended)

Raw RTF Text: { Apple0} (viewed in a text editor)

I expect { Apple0} to be {\b Apple\b0}, where \b starts bold and \b0 ends it, per RTF syntax.

Here’s a simplified version of my Python code:

import re

TERM = "Apple"
RTF_HEADER = r"{\rtf1\ansi\ansicpg1252\deff0\nouicompat\deflang1033{\fonttbl{\f0\fswiss\fcharset0 Calibri;}}\f0\fs22\par"
RTF_FOOTER = r"}"
BOLD_START = r"{\b "
BOLD_END = r"\b0}"

excerpt = "This is an Apple test."
term_pattern = re.compile(rf"\b{TERM}\b", re.IGNORECASE)
bolded_term = BOLD_START + TERM + BOLD_END # Intended: {\b Apple\b0}
excerpt_bolded = term_pattern.sub(bolded_term, excerpt)

with open("output.rtf", "w", encoding="utf-8") as f:
f.write(RTF_HEADER + excerpt_bolded + RTF_FOOTER)

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