In 2017 I filed a petition
here but I didn't have enough info to reproduce it reliably. Since then I believe I've been able to isolate it through trial and error during raids.
The issue seems to occur exclusively when "walk" is toggled, and then the character zones, and is then feared. The UI still shows "walk", and when not feared the character moves at the regular slow walk speed. If the character is feared now they will run at full speed.
You can work around the bug by re-toggling walk off and then walk on after zoning, which makes you walk when feared instead of running.
Zoning out is frequently used to heal up during fights like Klandicar, so I have to be careful to always re-toggle walk when zoning back out to avoid this bug.