Bug 1 
Navmesh is not being generated on brush components. I have a couple testing levels where I use brush boxes for floors. I checked the different levels and it is only generating on static meshes. I double checked and it is working in 5.4.4.
5.5
5.4.4
Bug 2
SpawnDecalAttached causes dynamic Navmesh to rebuild.
This was also OK in 5.4.4
Here is a video showing the bugs.
This bug has been driving me insane to no end. Everytime I load my project I have to move a brush and rebuild because the navmesh doesn’t generate on load. I get that BSP’s are just building blocks meant to block out a level, but this really needs to be fixed.
These bugs are still not fixed in 5.5.2 Release Branch
I did find two commits in the dev-5.5 branch.
I haven’t fully tested them, but at first glance they seem to fix the issues.
Bug 1
Brush box not affecting nav mesh - UE-235006
https://github.com/EpicGames/UnrealEngine/commit/0f86142448487b3594c20f8ae5e7e4066a27c832
Bug 2
Removed problematic navigation update when attaching component - UE-239292
https://github.com/EpicGames/UnrealEngine/commit/9d4a9385a11e6b37185137e3b25457a5796c6c5a
These bugs are now fixed in 5.5.4 Release 
