Quoting from Bluetooth documentation at Android Developers, it appears that this capability is based on connected device profile. Android supports this functionality from API level 18 (4.3). How to get Bluetooth Headset battery status in android from Stack Overflow indicate similarly, with vendor specific implementation links
Generic Attribute Profile (GATT)—The GATT profile is a general specification for sending and receiving short pieces of data known as "attributes" over a BLE link. All current Low Energy application profiles are based on GATT.
The Bluetooth SIG defines many profiles for Low Energy devices. A profile is a specification for how a device works in a particular application. Note that a device can implement more than one profile. For example, a device could contain a heart rate monitor and a battery level detector
Perhaps, ios ecosystem enforces this as mandatory and hence you can see it on your ios device
Since you do not wish to use a third party app like Potential Beta, it would be better to contact earphone OEM to ascertain
Edit I don't have marshmallow so am not sure if this setting tweak will help display-battery-percentage-android-marshmallow