I noticed the behavior you described in chord mode when researching another arp issue.
If you set your arp mode to up and play a chord (doesn't really matter which one) example does the first note play twice?
For example, when I play a C major chord (C3,E3,G3) I will get E3,E3,G3,C3. After that it will play as expected.
This behavior occurs probably 75% of the time. It happens in all of the arp modes except perhaps order and random (harder to tell in random).
Do you see this as well?