As @Angelicca said, in this case I’d still receive an email from Postcrossing! I’ve got such an email once, and that was because the account in question was deleted. (Very grateful for that, as the postcard had been travelling for months already, and I’m still sure that it was lost instead!)
Wow Paulo, thank you! Maybe it’s this? The user had 37 sent postcards, and only 7 registered. 10 postcards travelling to her were all registered on the same day – some of them had been travelling since March 2020 (a US → US card took 240 days to reach her, kinda sus…). I guess you just manually registered them?