File convert errors when importing mp3s

Hi, I have been having issues with converting sample rates when importing, can anyone shed light on this problem? The issue seems to happen only with mp3s, I haven't experienced it with WAVs. The infuriating thing is it doesn't happen every time with mp3s either so it can catch me out if I'm not familiar with the track I'm importing. I am trying to find the pattern in what's happening but it's a time waster in the middle of a session, so thought I'd post here in case there's a known bug?

Example (set up in a new empty project):

Project set to 48kHz, 24bit. 

Project settings > Assets : ticked both 'Copy audio files into project' & 'Convert audio file sample rate when recording'

When importing an audio file of 44.1kHz, I get different results if I drag the file into the tracks area of the main window, than if I use Shift+Cmd+I to import. 

Dragging the track into the main window DOES convert the file to 48kHz, but using File>Import does not. 

In my text example I imported the same file onto two different tracks side by side, using the 2 different methods, and the File>Import method produces a file of shorter duration playing approximately a tone higher...