What's going on with this Google-selected canonical?

I noticed something odd after implementing canonical tags on my site. Around the time of Google's December core update, I saw a significant drop in traffic and impressions—first on Google (to zero), then a week later on Bing. Traffic on Bing has since started to recover, but on Google, things are still flatlined. One major change I made during this period was adding canonical and self-referencing canonical tags. I'm unsure whether this was the cause of the drop, if it was due to the Google update, or perhaps a mix of both factors. When I checked some of my key pages, I saw something strange. For the 'Google-selected canonical' in Search Console, it pointed to a completely different website—one that I don't own, and that has nothing to do with my site or its content. This page isn’t indexed and is showing the message: *“Page is not indexed: Duplicate, Google chose different canonical than user.”* Is it possible that Google could select a canonical pointing to a completely unrelated site, especially one that I don’t own? I'm going to go ahead and try to fix this duplicate issue in the meantime. Thank you for the help and support everyone.