Expose on spawn for widget properties has been bugged for ages. At some stage the properties are (somehow) not loaded in yet.
*Edit linking my reports:
CreateWidget Bug - exposed property pins NULL on OnInitialized, useless.
Instead of running it on PreConstruct, can you try running the same code on Construct or OnInitialized? If that fixes it, I recommend not using the expose on spawn technique, but writing an initializing method for the widget to call right after the “create widget” node.
Also useful to know, PreConstruct runs in both editor and runtime. PreConstruct and Construct can run multiple times (any time the widget moves in the hierarchy for example). OnInitialized runs once at runtime.