The ongoing advancement of microelectromechanical systems (MEMS)-based chips for in situ/operando transmission electron microscopy (TEM) has enabled the investigation of the dynamic processes of oxide electronics at the nano and sub-nanometer scale under diverse stimuli, including electrical biasing, heating, cooling, and various gas environments. A critical aspect of this research is preparing electron-transparent lamellae from bulk devices, typically involving the in-situ lift-out focused ion beam (FIB) process. Traditional methods, using electron or ion beam-induced deposition to attach TEM lamellae to MEMS-based chips, often result in contamination and stray leakage currents, as evidenced by electron beam-induced current (EBIC) maps. This contamination can be prevented by employing a weld-free mounting method, which eliminates stray leakage paths and allows for accurate current-voltage measurements of oxide electronic devices. This study provides insights into how FIB sample preparation can falsely modify device behaviour and outlines reliable techniques to ensure proper nanoscale operando investigations.