Well, cause they are not compatible. For being compatible they have to be of the same class(or at least be derived one from another), not just to have the similar fields list.
The proper way would be to define structure in c++ first, expose it to blueprints, and then use it everywhere.
In case replacing bp’s structure isn’t feasible (like you saying “the project is huge”), you may create c++ structure similar to bp one (just like you did in a video) and then write a few bp functions for manually setting the fields of c++ structure based on bp’s one