I have a Ryzen 1800X running Debian Stretch with Buster's 4.12 kernel installed on an Asus Prime B350M-E motherboard. I was suspicious that the CPU wasn't boosting up to the higher clock speeds so I spent a while fiddling about in the BIOS before I found your post that explained why none of the usual reporting methods are telling the truth.
Running the BIOS "OC Tuner" caused the BIOS to overclock the CPU from 3.6GHz to 3.85GHz, but doing so appears to also disable the "Turbo-Core" boost. I only realised this when I couldn't reproduce your cpufreq-aperf results.
Disabling "OC Tuner" (which isn't straightforward since the only other available option is "keep current settings") caused the "maximum" CPU speed to go back to 3.6GHz. This re-enabled boost so cpufreq-aperf now reports CPUs running at over 4GHz.
I've had a lot of trouble finding reliable Ryzen on Linux information, so your post was very useful. Thanks.