Maybe in the Wattman settings. Right now it doesn't allow setting a fan speed lower than 10%. Does the hardware allow 0%? Can it be done?
According to AMD Wattman guide, Zero RPM is not supported on the RX5700 GPU Cards: https://www.amd.com/en/support/kb/faq/dh-020
Zero RPM enables quiet operation whilst the GPU is under light load and is enabled by default. The GPU fans should spin up as the GPU is put under load and temperature increases. Zero RPM is automatically disabled if any changes are made in Radeon™ WattMan.
NOTE! Zero RPM is not supported on Radeon™ VII, Radeon RX 5700 Series, and Radeon™ RX Vega 64 Liquid Edition graphics.
EDIT: Reread your post. I believe why the RX 5700 doesn't have the Zero RPM Option due to the amount of power and heat the GPU card generates while in use even during idle times. Just my opinion.