Recently, a methodology for fabricating polycrystalline metal-organic framework (MOF) membranes has been introduced - referred to as interfacial microfluidic membrane processing - which allows parallelizable fabrication of MOF membranes inside polymeric hollow fibers of microscopic diameter. Such hollow fiber membranes, when bundled together into modules, are an attractive way to scale molecular sieving membranes. The understanding and engineering of fluidic processing techniques for MOF membrane fabrication are in their infancy. Here, a detailed mechanistic understanding of MOF (ZIF-8) membrane growth under microfluidic conditions in polyamide-imide hollow fibers is reported, without any intermediate steps (such as seeding or surface modification) or post-synthesis treatments. A key finding is that interfacial membrane formation in the hollow fiber occurs via an initial formation of two distinct layers and the subsequent rearrangement into a single layer. This understanding is used to show how nonisothermal processing allows fabrication of thinner (5 mu m) ZIF-8 films for higher throughput, and furthermore how engineering the polymeric hollow fiber support microstructure allows control of defects in the ZIF-8 membranes. The performance of these engineered ZIF-8 membranes is then characterized, which have H-2/C3H8 and C3H6/C3H8 mixture separation factors as high as 2018 and 65, respectively, and C3H6 permeances as high as 66 GPU.