Automatic volume monitoring was introduced for the iPhone with iOS 14 and higher.
This feature is supposed to protect the ears by lowering the maximum volume after a certain period of time.
This actually useful feature can be irritating and lead to the assumption that the headphones are defective if the automatic volume adjustment is not recognized as such. However, the monitoring can be adjusted and – at your own responsibility – also completely disabled. Please test whether the maximum volume of the Teufel headphones is limited by this iPhone function before reporting a defect to us.
For more information about automatic volume monitoring, please visit the corresponding Apple support pages for your iPhone model.
Link tip > Further information about the "Automatic volume monitoring" feature at Apple Support