Swing added unexpectedly

[niksilver]

Describe the bug

1. Start up zxcvbn and hit Ctrl-8 to get to an empty track with mx.synths.
2. Enter f e f e and hit Ctrl-S and Ctrl-P to save and then play the track.
3. Notice the two notes are played with an even rhythm, as expected.
4. Hit Ctrl-P to stop the track.
5. Edit the track so that it's now (f e) (f e) and again hit Ctrl-S and Ctrl-P to save and then play the track.
6. Notice the two notes are no longer played with an even rhythm. The f is shorter, as if there is some kind of swing.

There is further unexpected behaviour if you change this last pattern to (f e) (f e) (f e) - some f e pairs have even rhythm and some have swing.

I'm happy to be told this is my misunderstanding of the syntax.