Hi B16 KJR, I can confirm there is a problem with the later Window 10 builds, although I am not running Build 1903, one of my machines is running a very late preview (20H1 Build 18917).
The same problem exists on this machine it cannot copy larger files. I tried to copy a 1.8GB file to the q10 and I get the not enough disk space error.
Disk space reports are all over the place for the samba drive. They do report correctly for Windows machines (both remotely and local).
You can try this on your machines, you will get different results between builds 1809 & 1903.
Got to a command prompt and map a drive to the Q10, I mapped z drive. At the prompt type (you can use your IP address) eg: net use z: \\192.168.x.x\share
Now type at the prompt:- wmic logicaldisk get size,freespace,caption
With the drive still mapped, you can also try this from the PowerShell Prompt:- get-psdrive | Where Free*
Going into the share in file explorer and selecting properties on the folder does seem to give the correct used space.