ParticleSystems created with the built-in pipeline that are “without user material set” will not be upgraded to default material of URP (Universal Render Pipeline/Particles/Unlit) and will remain “Particles/Standard Unlit”.
As a result, it will be drawn with the error shader (MAGENTA).
I forgot to write the version. It is Unity2020.3.17f1.
I also reported it as a bug report.
How to fix :
Create one material equivalent to the default.
Set “Default-Particle” as the texture and “Universal Render Pipeline/Particles/Unlit” as the shader.
(ColorMode = Additive seems to be good.)
Then assign that material to the particles that are in error.