OK ..so you have the RAM in the correct slots on the board ..now put both sticks on the same channel and see if your available RAM changes. This has to be a dead channel problem on the motherboard..I have run into this before. I also had this situation when a customer did not mount a Ryzen CPU all the way into the socket and it ended up burning some of the pins and that was the end result...one dead memory channel.
I have a new Ryzen Mobo (ASRock B450 Pro4M) setup and experienced the same thing except I had 4x DDR4 modules to play with which worked great in other Ryzen builds previously.
Installing the 3000 MHz pair gave what you experienced above, 8GB but only 3.5 GB available. When when I swapped them out for the 2800 MHz pair, the memory was recognized at 8GB available. So the 3000 MHz pair must be screwy, right? No:
The #&%$@ memory controller on the Mobo I guess is crap as all I eventually needed to do is keep mixing and matching the modules until one interleaved pair registered as 8GB available. Then I installed the other pair and I finally have 16GB available. Something like this:
Install 2x 2800 pair - 3.5GB available
Install 1x 2800 - 4GB available
Install second 2800 - 8GB available
Keep the 2800s, install 2x 3000 pair - 16GB available, success!