This question came up in the Online 10-day Retreat Nov 3, 2020, guided by Delson Armstrong. Day 2 was part of a daily 30 -minute discussion on the suttas, the Dhamma talk and reflections.
Slightly edited to improve readability
The interesting thing about the Satipatthana sutta - the four Foundations of Mindfulness - is that within each Foundation there are different ways to develop mindfulness. And in all four categories, in all four Foundations, you will notice there is the mindfulness of the arising and passing away of the feeling within the body, of the sensory experiences. There is the arising and passing away of the processes of the phenomena, that are happening within and outside of the body. There is the arising and passing away of the thoughts.
So within the context of the meditation, you are applying mindfulness to the extent that you are aware, or you are observing the object. You are just staying present with the feeling, whatever the Brahma Vihara might be. As you're staying with it, you might see thoughts arise and pass away in the background. Your awareness is so open, your mindfulness is there to the extent that you can see these things, but because you're not so fully focused, you're not suppressing the ability of the mind to be able to apply this observational power.
That's one reason why I translate, for example, mindfulness as observation, because observation is all about being aware and observing all of the phenomena that are happening, with this unification of mind around the object. While the mind is unified around the object, while it's aware with its attention around the object, it's still mindful of things that might arise in the way of hindrances, or insights that might arise, or what kind of factors might be present.
It's not to say that you're looking for it. That's the bare knowledge, that's the bare awareness which is; it arises when it arises, and it comes into your field of knowledge when you notice it, when you see it. But only  to the extent of you seeing it, not  looking for it, not trying to find it. It will come to you, as long as you keep your awareness  open.
Watch it here