OK seems like the crux of the issue is that dealii library uses taskflow library, and creates tf::Executor class during DLL loading / initiailization.
Then tf::Executor tries spawning workers, and waits for all workers to be created… But it looks like workers might not be created, creating a deadlock. Anyway, will try looking into it a bit more - seems like tf library is just not compatible with the way Unreal Engine loads the library.