I have a similar problem with my two monitors, but one matches closely to what AirIQ site lists for my district/area when I measure outside our villa.
Running hepa on high at night and reading is about 3 to 4 in the morning...but will increase when cooking, vacuuming the floor, doors opening/closing, etc. to about 12 to 15 on PM2.5. It quickly reads back to <5 after activity that caused a spike.
These are the two monitors I bought on Lazada. One is out of stock, the other no longer available.
The Xiaomi was unreliable and was a wasted 900 thb, white color. Xiaomi is junk in my experience.
The other one, black color, gives nearly identical readings compared to official monitors...799thb, but out of stock. I think its a good value if available, and reliable readings. Two years constant use/always plugged in.