The original reporting this is based on is by Blacklock’s Reporter [1] which looks like a better source, but unfortunately it's behind a login/paywall.
[1]: https://www.blacklocks.ca/205000-ballots-not-counted/
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PS: I found this older article [2] which covers the same issue. Apparently, if my understanding is correct, this number (~1/6 of the mail-in ballots) is much better than the 2019 election [3]. I guess that is something you could expect. Also, I'm not sure what a cancelled ballot means. Does it include the case when someone simply forgets to return the ballot?
[2]: https://www.mapleridgenews.com/news/1-in-6-mail-in-ballots-n...
[3]: Ibid. "Elections Canada says there is typically a gap between the number of mail-in ballots that go out and the number that come back every year. In the 2019 federal election, only 55 per cent of the domestic mail-in ballots and 61 per cent of international mail-in ballots were returned on time. The remainder were either returned late or not at all."