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 
I’m still getting this issue in instanced static meshes spawned using Blueprints. I updated the project to Unreal Engine 5.5.4 and still having the bug. I tested the same scenario in Unreal 5.4 and it works perfectly. Any suggestions please?
@Dazzling_Divine I may have a solution even though It could affect startup time when playing the level depending on how many of the blueprints are in the level. It should be a temporary fix and hopefully Epic can fix it. You should send a bug report to Epic.
I recreated your blueprint from the video (minus setting materials). I used the Shape_Plane mesh from the starter content. I was getting the same issue when reopening the level. The Blueprints were not updating InstancedStaticMesh components on the NavMesh.
The issue seems to be that the InstancedStaticMesh components are loading with empty bounds when the level is opened. I get this navigation warning when opening the level.
So for a test, I moved the code from the Construction Script to an Event named Build.
Added the Build Event to the Construction Script.
Then on BeginPlay I clear the Instances and rebuild them at runtime which seems to also update the NavMesh.
When playing the level if you hit the console key and type show Navigation you can see the NavMesh.
Even though this is not ideal, it should help until Epic can fix it.
I think Epic updated the code for Navigation in 5.5 for NavLinks to be generated and may have caused a few issues. lucky the two bugs I had got fixed in 5.5.4.
Thank you very much for your detailed explanation and suggestions! I really appreciate the effort you put into testing this. I’ll try your method and see how it works in my case. Hopefully, Epic can address this issue in future updates.
