I have switched back to Liquid rom (Oreo) because the battery seemed to drain faster too.
but which magisk model do u mention? and stock camera means stock from this rom or the original Mi A1 camera?
Stock original Mi A1 camera.
For me with a "normal" use the battery lasts me 3-4 days. I use Moun Kernel and optimize the phone configuration with LSpeed in its balanced mode.
Rubén