Clean install of Windows 11 a couple of weeks ago, and I opted to not restore old settings for many programs, including FDM, so clean install of FDM also.

Windows 11 23H2 (Build 22631.3447)
FDM Version 6.21.0.5639

I've enabled the "Notify me of failed downloads via Notification Center" setting, but both last night and again today, I've received Windows 11 Toast notifications of a download failing when in fact, it has not.

The last file I grabbed that it said had failed was a file from GitHub, called PixelFlasher, and I had grabbed it's .SHA256 hash file for comparison, and it checked out, so it definitely did not fail.

I also have FDM set to invoke Windows Defender after downloads are completed - could this be a factor?

Hello,

I think there were some failures during the downloading but FDM finally has managed to finish the download.

I think you're right. Because it happened again, yesterday, and this time I did see notifications where the download started, then stopped, then started, then stopped, and then finally started and completed.