Adding a content type not showing certain columns

I have two site content types A and B . In A there is one column "Subject" which is hidden and in B it is optional.

Scenario 1:

when I add content type A on library then column "Subject" is not added to library. after that when I add content type B to library then also, even if "Subject" column is optional in this content type it is not added.

Scenario 2:

First I add content type B on library then "Subject" column is added to library.After when I add content type A to library then that column is shown as hidden in B content type.

Is this a bug which is in scenario 1? Can we have any workaround or solution for that?