Low voltage warning
When the supply cannot deliver enough current (amperage), the
output voltage will drop:
tested with my iphone 11, seems works fine
Yes, but your iphone has an internal battery, and that is what it is running off of. Notice that the amount of time it takes to charge a phone is (usually) substantially less than the amount of time that charge will last last, point being that as long as the phone is plugged in, the charge on the battery is still increasing even if you are actively using the phone. It isn't running off the powerbank, it is running on it's own internally regulated power, which is provided by the battery which is charged by the powerbank.
While this implies the amperage from the powerbank is at least as much as the amperage used by the phone, we need to qualify this with over time. At any given instant, the amperage used by the phone may be greater than that coming from the power bank, even though averaged over time it is not.
The phone's battery thus serves as a sort of a giant capacitor. But a Raspberry Pi does not have a battery. When in use, like the phone the amperage consumed will fluctuate widely and rapidly.
Powerbanks are not designed (or sold) to power electronic devices and respond properly to that kind of instantaneous current draw. They are designed and intended to recharge batteries. These are very different scenarios. Delivering fast instantaneous current is not something you want happening with a lipo battery -- they can explode. But that is the opposite of what you need in a Pi power supply.
The take-away here is don't bother shopping around for a "better" powerbank. If you want to power the pi from a battery, you need some hardware intended for that purpose (there are hats, and suppliers like Adafruit carry various things).