When uploading audio files for a promo, if the watermarking pre-processor encounters an error, it will highlight the suspect track(s) in red.

MP3 files can get corrupted if there is an issue while encoding them.

Look at the suspect track's properties and confirm the bitrate stays set to what you initially encoded.  Try re-encoding the track and uploading it again.  If it's a variable bitrate, changing it to a constant bitrate can sometimes help too. 

TIP: Open the file's properties in Apple Music and if the Encoder lists as "Unknown," that's a clue that it should get re-encoded.