So apparently this happens with any variable on any child blueprint thats placed in any level, and this is 100% reproducible even in the 5.5 launcher version!
The following screenshots are from a blank project using the launcher 5.5 version of the engine (I have also confirmed this with our studios own 5.5.1 merge)
Here is the parent blueprint… it has a static mesh component with no static mesh set, and a text component with the text Base Class set.
Here is the child blueprint that has a static mesh set and the text changed to Child Class
Here is the child blueprint placed in editor in a default world partitioned map.
Here is that same level in a cooked, packaged build.
I don’t like to use hyperbole often when dealing with this engine, but in the 10+ years I’ve been working in Unreal this is THE WORST bug I’ve ever seen shipped. This makes the engine completely unusable in all cases and should have been caught by QA easily. In fact I have no idea how this actually got out to the public. Fortnite does not use a custom engine version and generally the mainline p4 stream matches what they’re using (I believe it lags a bit as they merge back and forth between streams). Assuming their unreal mainline is mostly up to date with FN, this bug would be present in FN which I just cannot comprehend being possible.