We can't identify the cause from the photos, but suggest you have a certified arborist assess the tree. We don't see much of a root flare -- a feature that is particularly notable on mature beech trees -- which suggests the tree is planted/growing too deeply or might have been over-mulched at some point. The stress from being buried too deeply (even a few inches can make a big difference) can result in various problems like girdling roots, stress that attracts borers or which may make it easier for certain wood decay organisms to enter the trunk.

Arborists can't cure or treat every condition, but they can help to narrow-down likely causes and look for sources of tree stress that might be able to be alleviated. Even if wood decay is present and associated with this symptom, it won't necessarily doom a tree, as trees sometimes successfully compartmentalize damaged tissues internally to keep decay from spreading.

Miri

You're welcome.
Yes, unfortunately trees being planted too deeply is quite a common problem; some trees tolerate it better than others.

Miri