You should have noticed that when you connect your iPhone charge, the battery indicator should be showing green colour with a lightning symbol, but it shows a yellow icon no matter how you try.
The reason the battery icon would remain yellow is due to wherever you enable “Low Power Mode”. So, when you disable the Low Power Mode, you will notice the battery icon turns to green colour with a lightning symbol while charging.
When you connect your iPhone charger to your iPhone, the battery indicator is supposed to show a green color with a lightning symbol.
However, under specific circumstances, there might be different outcomes. So, if your iPhone is on the Low Power Mode and then you connect your iPhone to a charger, then your battery indicator will remain yellow instead of green.
Therefore, you can disable the yellow color battery icon by going to the Settings > Battery > toggle to disable “Low Power Mode”.