I’ve always been confused about why sixth-generation signals (see: History of fire alarms, by generation]History of fire alarms, by generation]), made after the ADA was passed, often have flaws like this - i.e. non-syncable and low flash rates.
Though I will say that the nominal flash rate is not always the actual flash rate. The 4904-9501 is rated for a low flash rate, but usually flashes faster than 1 Hz.