Tipped arrows with instant damage effect don't work well

I tried to shoot the ravager with tipped arrows that has instant damage lv2 and a bow enchanted with power lv5 and flame.

In theory, I only need three times critical charge shooting with lv2 instant damage arrows and bow with power lv5 and flame ((9*(1+50%+25%*4)+12+5)*3=118.5>100) to kill the ravager since the damage from the arrow itself WILL STACK with the damage from the instant damage effect in bedrock edition. And only after the fire on the ravager is extinguished naturally (not by water or rain) , I shoot again.

However, I actually need 4 times critical charge shooting with lv2 instant damage arrows and bow with power lv5 and flame to kill the ravager in the game. The ravager does not enter or touch the water or rain after it is on fire and only after the fire is extinguished naturally, I shoot again.

I have also tried to shoot other mobs with 20 hp with bow enchanted with power lv5 only and all the time I can kill them with only one shot. And I know that power 1 will increase the damage by 50% and 25% thereafter for power level greater than 1 and the base damage of a critical charged bow is 9-10 in bedrock edition. I have also tried to shoot the mobs with only 10 hp and I can kill them with one shot and one melee attack without using any melee weapon. In addition, I tried to kill those mobs with only 10 hp with only melee attack without using any weapon and found that I need to hit them 10 times to kill them, meaning that melee attack with empty hand delivers 1 hp damage only.

I am wondering whether the invulnerability mechanism is change or are there any problems?