The CPU is doing what it's designed to do - operate at lower clocks at low load for energy efficiency and then boost up when workload demands increase to maximize performance. You paid for a CPU that can be high performance when workload requires, and still be power efficient at lighter loads, so why would you want to castrate it by setting a fixed frequency?
CPU's running at a constant clock are a thing of the past (yes, even Intel CPU's these days do not run at a constant clock unless you force them to). If the clock speeds changing is something that bothers you, then don't monitor the clock speeds lol.
With all that out of the way, if you absolutely want to lock the clock speeds, you can certainly do that. Either by setting a manual overclock in BIOS or Ryzen Master by locking frequency and voltage (you have to figure out a stable combination), or in the Windows power plans by setting the minimum and maximum processor states to be the same percentage.
By locking frequencies though, you are either sacrificing power efficiency or maximum performance depending on what you set them to.
Otherwise, just leave it alone and let the CPU manage itself. You've given it good cooling so it'll be happy.
