Static mesh navigation collision only saves one property at a time
When trying to add navigation data to a static mesh it only seems to save the first thing that you change each time the mesh editor is open. I circled what I'm talking about in red in the following screenshot in case it's not clear what I'm talking about:
As an example, I added a box collision to my mesh and set its extents to (200, 200, 200). After saving it and closing/reopening the static mesh editor the box was there with its default values. I set the extents again, saved, closed the mesh editor, reopened it and the extents were (200, 0, 0), repeat 2 more times to get the correct extents. This also happens if you change any of the other navigation settings like "Area Class" or "Is Dynamic Obstacle", whatever the first one you change is gets saved and everything else is ignored.
For repro steps:
This seems similar to this bug but that's marked as fixed and still happens to me in 4.18
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