How do I make a high signal last longer so my nodeMCU/Tasmota sees it?
As the comments suggest, my problem stemmed from using an analog input signal linked to a Tasmota rule. Either Tasmota wasn't polling the input value quickly enough to detect the change, or the photoresistor wasn't reacting quickly enough to the brief surge of light.
I replaced my LED-strip/photoresistor with an optocoupler and it now works reliably on even the briefest of bell pushes.
Many thanks for those who pointed me in this direction.