Thanks for providing this tip!
The information in the "view plan > data section" is accurate (mine shows 97 minutes left). Thus it looks like a backend problem related to #MIN# as both My Account application (iOS and Android) as well as the main My T-Mobile account summary page, aren't properly communicating with the database.
This needs to get passed on to their dev team given it's been going on for awhile now (most folks like myself haven't noticed it before since I rarely have a need to login to My T-Mobile - I only noticed this when I signed to change my device type).