REGRESSION: Yes, does not occur in 4.17.2
Navmesh does not automatically update when the user undoes the scaling of a nav mesh bounds volume.
This issue occurs with both changing the scale in the Transform settings, and with changing the extents of the brush.
Navmesh does not update automatically
Rebuild paths in the level.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-51013 in the post.