When you do a 'tablespace' mode export, you are essentially moving the tables that exist in those tablespaces and any corresponding indexes. It is essentially a table mode export and materialized views are not moved in a table mode export.
What you are seeing is by design and not a bug. Not necessarily what you want to happen in your scenario. You will have to create a list of materialized views and move them in a separate expdp/impdp set of jobs.
Thank you for reply, Dean. But if this is the case, I think it would be better not to create the tables bearing the same names as the MVs. Right? Otherwise, I have to drop those tables before I recreate the MVs. To me , it looks like Oracle expdp/impdp cannot differentiate MVs and tables. Is there any Oracle document to back what you said?