Maybe these steps will help reproduce.
I use my laptop plugged in via HDMI with a USB-C hub (keyboard/mouse/headset).
If i plug in my charger the CPU drops to 400MHz, i then have to use a gnome plugin called “CPU-Z” to tell the CPU governor to use 100% of the CPU, everything is fine then.
If i then let the laptop go to sleep for around 15 minutes (eating lunch) and return, when i wake the laptop, the fan then spins at 100% and the CPU is then throttled to 400MHz, however this time the “CPU-Z” plugin can not recover this, so i have to unplug the power adapter & HDMI, shut the laptop lid and let it go into hibernation.
When i then open the laptop, the fan is throttled down again and “CPU-Z” can then tell the governor to use 100% of the CPU again.
I hope this information can help.