I found a teacher's edition PDF that discusses how to present the brief article by Mariko Asano (reprinted from Oxford Selected Readings Pre-Intermediate) about her experiences with the nonprofit organization Habitat for Humanity in the Philippines. Here is the text (with the cues for teachers removed):
I am 24 years old, and I grew up in Nishinomiya, Japan. Several years ago, I went to Negros Island in the Philippines as a Habitat volunteer. This was the first of three trips I have taken to the Philippines as a volunteer. For me, the idea of building somebody's house abroad was very exciting. The next year I returned to Negros Island as a Habitat volunteer. This time I went as a student leader with 28 classmates from Kyoto University of Foreign Studies.
Both the staff and the families on Negros Island became dear friends of the work team I led. Meeting these people was wonderful for each of us. Their lifestyle reminded us of the meaning and value of life. The people also helped us appreciate the more valuable things in life, such as spending time with your family, friends, and neighbors; developing close relationships; helping each other; and appreciating what you do have. These things are sometimes forgotten in an affluent country like Japan.
We thought we came to the Philippines to help the Filipino people, but they helped us to see something valuable. They generously offered their food, space, and hearts in a way we were unaccustomed to. (Would you give up your bed for a stranger and sleep on the uncomfortable cement floor at your own house?)
When I took my third trip to the Philippines as a Habitat volunteer, I was assigned to a house with young people from around the world. In my group, there were Filipinos, Americans, Indians, Koreans, and Japanese. We worked together to complete a house for a family we met on the site. On the last day, all of us stood inside a room we had built in just a week, feeling a sense of fulfillment. Even now we keep in touch across the world. Some of us are actively involved in Habitat in different countries.
Habitat brings people together and helps us realize that people all over the world care about each other. Habitat sends the very important message that we can all be friends. Being involved with Habitat for Humanity has changed my life. I've learned that I can make a difference in the world.
The second paragraph is the relevant one for your question—and the only way that a native English speaker could reasonably understand it is the same way you did: If the true/false question on this material was stated as "Mariko thinks that Japanese people always appreciate what they have," the correct answer is False.
A grader may mark a True/False answer incorrectly for a multitude of reasons—everything from misreading the sense of the question to accidentally making the wrong mark on the test page. But in this instance your understanding of the meaning of "sometimes forgotten" is correct, and your response to the test question should not have been marked wrong.