Notes missing from MP3 and WAV exports

I have a three-minute piano piece that plays fine in Musescore. In the exported audio to MP3 and WAV, two single notes are missing (not audible). The same notes (tones) when played elsewhere in the audio sound perfectly. Repeated export attempts continue to lack the exact same two notes. Sound font is 4U Mellow Steinway. Again, when played in Musescore, the two notes are perfectly audible.