The pronunciation is highly variable and the spelling has historically also been variable. When French words are imported to English, sometimes people try to retain the French pronunciation and other times they anglicize it. This word seems to have been handled both ways.
Another thing that happens is that both English and French change their pronunciation over time. After English imports a word, the French pronunciation may change making the English word look odd or not even look connected. Not sure that this happened to “timbre” but it did happen to words like “chief” and “chef”. Both were imported from French but at different times. “Chief” when French used the hard ‘ch’ sound and “chef” when French had switched to the soft ‘sh’ sound.
Another thing that happens is that both English and French change their pronunciation over time. After English imports a word, the French pronunciation may change making the English word look odd or not even look connected. Not sure that this happened to “timbre” but it did happen to words like “chief” and “chef”. Both were imported from French but at different times. “Chief” when French used the hard ‘ch’ sound and “chef” when French had switched to the soft ‘sh’ sound.
https://www.merriam-webster.com/dictionary/timbre https://en.wiktionary.org/wiki/Timbre