Backblaze skips folder Render Files -- but only sometimes!!! How? Why?

I noticed a 43 GB file I was doing a test restore of from Backblaze came back at about 20 GB. The Render Files folders were missing.

But I recently tested restoring a 1.26 GB FCP library file. And the folder was present!

What's going on? Does Backblaze know about the Render Files folders, and only skips it for large libraries?

This is a mixed blessing, as it speeds up transfers, but re-rendering my main movie takes a very long time due to the fact that I am using noise reduction, and dust and scratch removal.

Can anyone here explain this? TIA.

FCP 10.4.6

macOS 10.14.6

Backblaze 7.0.2.484