During an inspection it was observed that we used to carry out the method with a mixed mobile phase solution using two different lines from the pump system instead of an unique line from a previous mixed solution.
EP and USP still ambiguous about how to prepare the mixture to use in the system.
Could someone help me in the interpretation of the official monographs?

When you use a premixed mobile phase, just be aware that the more-volatile component can evaporate off over time, changing your separation conditions, retention times, etc. As you state, sometimes (oftentimes) USP and EP are not clear (for example, USP 25 glycerin monograph details using a solution from a just-deleted section), just make sure you document what you actually do on your instrumentation. Remember, USP allows equivalent test methodologies to be used, as long as they are validated. And there are ranges for mobile phase composition, flow, temperature, etc. that would be considered so minor that re-validation is not necessary, considered tweaking. Just document.