Thanks for the report. I’ve added this issue to our bug tracker. (This is one of those things that is technically working properly, but which should definitely be improved: The time that you see there isn’t the length of the file, but the current position of the waveform display’s audition cursor, which defaults to being at the file’s start. As you’ve guessed, it really should default to the end of the file so that it shows the duration of the file when not playing.)
As a workaround, you can force the waveform display to show the correct length by clicking on any part of it. This will cause the waveform to audition from that point. Once it reaches the end of the file, it will display the correct file length.