Is it normal for MacBook M1 battery to lose charge when connected to power?
If the power adaptor cannot cover the load then the difference is taken from the battery.
Once the load is reduced then the battery gets recharged.
Seen that on my macbook pro and it also depends on the size of the charger. So I can use a 65W charger and see that behavior, but when I connect my 85W charger it rarely happens.