Drinking water has emerged as an important route for microplastics (MPs) to enter the human body, prompting concerns about their adverse health impacts. Membrane filtration technology is widely recognized as an effective treatment solution for combating MP pollution in water. However, recent research disputes that polymeric membrane systems may serve as additional sources of MPs in drinking water. The aim of this research is to investigate MP release from ultrafiltration membrane systems under different operating conditions by providing concrete evidence, identifying the operational factors contributing to the release, and elucidating the underlying possible mechanisms. Two key pieces of evidence were found to support the assertion that MPs were released from membrane systems, i.e., negative removal efficiency and an alteration in MP compositions observed between feed and permeate samples. Surprisingly, the MPs released from the membrane system originated not only from the membrane material and its additives but also from plastic-made equipment and even the other polymers used in the system. Overall results reveal that destructive activities such as shear stress, mechanical abrasion, and chemical oxidation processes, along with the carrying of MPs from external sources, are identified as potential mechanisms driving the concentration increase and polymer composition shift of MPs in permeate water. This study enhances an understanding of MP pollution in drinking water caused by membrane technology, potentially spurring the development of mitigation strategies for this issue.