The reason the FWAA calls for Flat Tops is because they are shorter than many button tops. Flat tops are usually Unprotected, and button tops are usually Protected.
The FWAA UI has built in LVP. Flat top UnProtected allows the light to reach maximum output, without tripping a battery protection circuit.
Unprotected cells have higher discharge rates (10A), than Protected cells (usually 3A). So for max output, flat top unprotected, high discharge, will outperform a protected button top.
Protected cells are longer, this may prevent the light from closing far enough for the inner signal tube to make contact with the driver.
Using Protected cells in a High Drain Light, can trigger the Battery protection circuit, so the light suddenly shuts off. After battery protection is tripped, the battery reads 0 Volts until the protection is reset, by charging.
so, use UnProtected cells in the FWAA.. even if they are Unprotected button tops, as long as the battery does not exceed 50.5mm in length, they will work in the FWAA.