After updating to version 2.4.2, every MP3 I open/import has the first second or two of the audio cut off. This happens with all MP3s. I’ve confirmed it does not happen with flac or wav files. Not sure what to do at this point other than reinstalling 2.3.x.
As a work-around you can try converting “problem MP3s” to WAV with a different application. Audacity decompresses the MP3 when you open it anyway so technically it’s no different. It’s just an extra step for you, and it may get-around a problem.