HISM is generated by ConstructionScript Actor is placed on the level and navmesh is generated on top of HISM. In UE5.2, the navmesh was generated without splitting on the HISM. In UE5.3, a break occurs in the center and the navmesh is not generated as expected.
Open the repro project in editor with UE5.3
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-198801 in the post.