The Sera NX040 datasheet specifies a power supply range of 1.8V-3.6V with 3.3V being typical. Are there any conditions that would require a higher voltage than 1.8V?

The module works down to 1.8V and there are no technical issues with using a 1.8V power supply. However, designs may want to consider setting the lower voltage limit as 2.0V as it is possible that a UWB current spike could occur when the host or BLE radio is doing some other function which could potentially cause a brownout/reset on the UWB chip.