Problem with 64GB of RAM in the form of 4 x 16GB modules showing up as only 32GB (31.xGB useable) was finally resolved when I followed this seemingly strange procedure on youtube:
It basically changed a single setting in the registry; then check that someone has not told "msconfig.exe" to use a limited amount of the available RAM (a legitimate process used by software and hardware testers and developers); then swapped the DIMM order.
I had the same problem with running all these 3 OS's: Windows 7, Windows Server 2016 and Windows 10 Pro, so I think this is mainly associated with the DIMM order or imperfect seating causing some issue. Perhaps it is to do with two slightly different batches of DIMM's. I initially ordered 2 x 16GB (32GB) installed them and then added another 2 a few weeks later from another source, without considering the pairing in appropriate corresponding slots.
Motherboard: GA-AX370-Gaming K7 (rev. 1.0)
DIMM's: CRUCIAL 16GB 2Rx8 CT16G4WFD824A.18FB1 DS Micron CL17 1.2v 2400MHz
These were selected from the Supported Memory Test List for their ECC support capability on this non-server motherboard.
For details on how ECC works with Ryzen refer to this very good article
Others having the same or similar problems