@Mathiason I feel like its more than just common sense. He has memories of 3 lifetimes. A grown up, a swordsmaster and a wise mage. Even with the explanation of it being memories, but not all the experiences, his dumbness feels a bit forced.
Well considering he knows he's op, he might just not care too much about "small stuff" and just pretends to lack a bit.
Either way I that's one of the annoying parts of those isekai or other series, with op but dumb chars.