QStorageInfo.mountedVolumes() does not detect volumes mounted via /etc/fstab or /etc/auto_master. As far as I can it only detects network mounts created via Finder.
Here is a link to the relative section of code.
The problem is in the option kCFURLEnumeratorSkipInvisibles. Using virtually any other option in the CFURLEnumeratorOptions results in a complete listing of mounts.
I was able to recreate the Qt behavior in a Swift playground file with the following contents
Here is a link to all the possible options that CFUrlEnumeratorCreateForMountedVolumes can be called with: https://developer.apple.com/documentation/corefoundation/cfurlenumeratoroptions
I suggest a different option is considered such as skipPackageContents.
Another quick sanity check is to mount a network share via /etc/fstab or /etc/auto_master and then run mount from the terminal, and compare that to the output of QStorageInfo.mountedVolumes().