I think it's because the issue seems specific to image attachments. I can't reproduce this with non-image attachments, but can consistently repro with image attachments from a few threads I've just tried. Maybe there are now more image attachments (eg. recent threads on tilesets and such) so the bug becomes more noticeable?
I found that with image attachments, the bug seems to be that the download link alternates between the 404 and the success states. Just pick a suitable download URL and retry it multiple times (you don't have to download, you can cancel it at the point the browser asks you whether you want to download or not, if your browser is configured to prompt first). This state seems to be per link--if you try it with two different download links alternating between them, you get 404 (link 1), 404 (link 2), working (link 1), working (link 2), rather than 404 (1), working (2), 404 (1), working (2).
Not really sure what next steps to take though to try to fix this. Do we even think it is a recent problem, or is it possibly just more noticed recently because maybe there were more image attachments? The 404s don't seem to produce any entries in the error logs in Admin Center either.