The operational amplifier should be able to use another current, which is not a bandgap referenced one, constant with temperature. A PTAT biasing current is better for BW stability with temperature. At least, it is easier to design an opamp with a PTAT. And, if the bandgap referenced current varies also with resistor corners, the specs for the opamp are harder.

That implies two different bias generators, one bandgap-referenced and one PTAT-referenced, since there are multiple amplifiers so the PTAT current will need to be copied and distributed across the chip.