Can cis and trans isomers have widely different flash points?
It is not totally surprising giving that they are stereoisomers. Think about maleic and fumaric acid, which due to their very different properties were given different names.
Moreover, but perhaps you are aware of, there is not in general a relationship between boiling point and flash point. In the first case factors which depends on the actual configuration - as molecular symmetry, polarity - should be considered for how they affect intermolecular forces. For the flash point, their influence to the intrinsic stability of the molecule and to the overall shape and accessibility for oxygen, should be considered instead. 

For the less experienced ones: in few words volatility is required to reach the concentration (in air) at which the vapour can be ignite at a given temperature (flash point). More a liquid is volatile, easily that concentration is reached. But this does not imply a lower flash point, which depends on the thermodynamics and the kinetics of the reaction between the vapour molecules and oxygen in air.