I did some digging into this recently and had a lengthy session with QB support. Shared my screen and replicated the issue.

When you click into an Invoice or Estimate, you'll see what I think is a transaction ID in the URL. It's a 5 digit code that is sequenced based on new INV or Estimates created. The PDF/download is then defaulted to use that 5 digit transaction ID code for the naming convention.

The support agent was supposedly talking with their back end people and they couldn't figure out it, or come up with a solution. I was told they will work on it.

It's a major blow to the efficiencies of this program, for a business that downloads every invoice for file keeping purposes and for emailing to our clients. if it isn't fixed, we'll be moving away from QB for future invoicing.