I've been listening to a podcast that got into this history of Miyamoto Musashi. Several of the duels described talk about Miyamoto Musashi using a wooden sword -- in one case even taking the time before the duel to carve a new wooden sword from a boat oar.
Why he uses a wooden sword isn't addressed in the podcast. Was it common for one or both Samurai to use a wooden weapon in a duel? Is there any reason for wooden weapons to be used?
If you're curious about the podcast -- I have no affiliation with Our Fake History:
https://ourfakehistory.com/index.php/season-3/episode-55-who-was-japans-greatest-swordsman-part-i/
https://ourfakehistory.com/index.php/season-3/episode-56-who-was-japans-greatest-swordsman-part-ii/
This question actually came up here a month or so ago, and got an interesting answer from u/wotan_weevil. While there's always more to say, you might like to review that response while you wait for fresh responses to your query: