These symptoms do not appear to be caused by a lawn ailment (especially since we don't see much if any desirable turfgrass in these photos, though we realize you may be using "lawn" loosely). The appearance of the damage is not consistent with Fall Armyworm but could be due to one of several common pathogens that cause similar patterns of dieback. Alternative explanations include  localized animal urine exposure, spot-treatments or spills of herbicide (since some of the foliage in the first photo looks "burnt"), or perhaps even physical damage, like a wild animal (such as deer) bedding-down for a few hours and squishing plant crowns.

Wet weather or frequent irrigation can support the development of "blight," "crown rot" or "root rot" fungi, collective terms for diseases caused by several separate pathogens. The exposed soil does look wet and compacted, though we don't know how long after a recent rain you may have taken the picture. Poor and damp soil would also be a reason for turfgrass' failure to thrive in this area. (Plus, if it's too shady, a different groundcover planting would be more suitable and successful than lawn in such areas.)

This page provides a little bit of info. on this grouping of diseases, and while it is geared towards container plantings and decorative flowering plants, these pathogens can infect weeds and other groundcovers as well: https://extension.umd.edu/resource/root-crown-and-stem-rots-flowers

Little is discussed in the above page about control options for in-ground plants because there isn't much you can do. Pull up and dispose of any dead growth and try to air the area out by reducing irrigation (if applicable) and consider amending the soil and raising the grade of the soil (perhaps by constructing a raised bed) depending on which plants you intend to grow there. If a roof downspout empties nearby and floods this area, redirect its output.

A much simpler alternative is to embrace the conditions present and simply switch plantings to species that are known to thrive in poorly-drained or damp conditions; native species are ideal, but any non-invasive candidate should suffice. How much summer sun the area receives coupled with considerations like deer browse risk will help narrow-down what could otherwise be an extensive list of candidate species.

Miri