В общем, как тут пишут, действительно этот столбец надо было апдейтить
» In case you had records before the creation of the inheritance relationship, you should go to the database and modify the value of the Discriminator column to an appropriate one, otherwise, these records will not come out in your queries
Ну да, в ссылке же выше "By convention, EF will not automatically scan for base or derived types; this means that if you want a CLR type in your hierarchy to be mapped, you must explicitly specify that type on your model. For example, specifying only the base type of a hierarchy will not cause EF Core to implicitly include all of its sub-types."