I managed to get a response with some transparency here:
Netlify don’t believe it to be a re-occurance of the bug that prompted this thread, but simply that the “Drag & Drop” feature is inherently prone to failure.
It’s dependent on things like the size of the project being uploaded, your browser memory and the stability of your network connection.
The worst part of its behavior is that it doesn’t provide a timely contextual error message when it does fail, so you can end up stuck on a spinner for an extremely long time, and then you aren’t guided as to what went wrong or how to fix it.
As Netlify consider it the “least-desirable and least-capable upload method”, they’re unlikely to improve it, so if possible you really should use the Netlify CLI, which is a core part of their system and likely to be much better supported.