Decathlon inner tubes go flat

I got an inner tube replaced at a local Dr Bike about a month ago. IIRC, they used this one from Decathlon. The tyre says 700x38C so it should be all good, right?

Well, the other day it went flat. I inspected the tube thoroughly and even submerged it in water but saw no bubbles. An inspection of the tyre didn't show any obvious cause.

I decided to replace it with a new one just in case, of the same model. This time doing it myself. Used it for a day and it went flat again.

To add insult to injury, both the tyre and the wheel were replaced only like a year ago at a great expense!

What's the likely culprit? Has anyone had a similar issue with these tubes? Or perhaps I should look elsewhere?