Yet again today it has been the proof that stepping back to have a more detached view can be really valuable.
When you work on something on a daily basis for some time you get accustomed to it and its quirks. You stop looking at inefficiencies and problems as such and you start looking at them at just how it works. The most glaring problems are going to be obvious, but the ones that are sneaky and look fine for most people are going to pass through your filter.
After a year of working on a project we had an external eye take a look at it and they found so many visual and UX problems that we just didn't notice because we are so used to do things our way.
Our way was not wrong, but their way was not either. They were just using the app as a first time user would. Bonus points if they're not really tech literate.[^1]
You need to make them use the app without any interruption or supervision. You need to be an onlooker. You need not interact with them until they finish and the most important thing you need to stop yourself from trying to correct their behavior if they do something that deviates from the usual way of doing things.
[^1]: You can't assume that the entirety of your userbase comprises of tech-literate people. A lot of people will be, but don't expect that it's a given.