I noticed that after updating the bios on my AMD B350 motherboard to AGESA version 184.108.40.206 that it stopped reading my installed RAM as 16GB. Now windows reads installed RAM as 15.9GB. Before anybody says its a windows 10 thing, any bios version before 220.127.116.11 reads it as 16GB, so it's not a windows 10 issue. Some change in the bios starting with AGESA 18.104.22.168 has caused this issue. Subsequent updates since like AB, and ABB have not remedied the issue, so my question is what happened? Why the change?
See what new BIOS settings for RAM Memory has changed.
Plus showing .1 gbyte less of RAM Memory isn't anything to worry about. It could, as you mentioned, a new BIOS setting causing the issue.
Open a Support ticket for your Motherboard since they are the ones the creates and updates their motherboard BIOS version and see what they say.
You can also open an Online AMD Service Request from here and ask them: https://www.amd.com/en/support/contact-email-form
After every BIOS update you should choose the load optimized default option in your BIOS and save the changes made. Then you will see 16gigs of RAM. I had that same issue and I have fixed it this way.