When measuring the flash point, why does no further flashing occur after the initial flash appears?

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I have recently started working with the analysis of oils. When determining the flash point of an oil additive, per the standards, heating is continued after the first flash, and the experiment is considered valid only when a flash occurs again. I would like to ask everyone: what is the necessity of re-ignition as specified by the standards? Also, during the analysis process, the same sample can sometimes experience a secondary flash fire. Sometimes, there is no further sparking, and during ignition the flame at the nozzle does not go out; however, there is also no obvious sparking, giving the impression of a state where combustion has not yet occurred. We use the closed-cup flash point determination method, with a closed-cup flash point tester (produced by a factory in Dalian). The composition of the samples used for testing remains relatively constant, and their flash points are in the 80s degrees, showing good stability. When the above situation of no further flashback occurs, the initial flash temperature is relatively high. The operation follows national standards; the heating rate is roughly 1 degree per minute after reaching 50 degrees. Ignition usually begins at around 60 degrees. I was wondering what usually causes this phenomenon to occur? Are there any issues with the operation? :Could everyone help analyze the handshake? Thanks!