This is 100% reproducible behavior.
Here’re the steps to reproduce:
- Create a 4/4 measure with (for the sake of simplicity) four quarter notes.
- Select 2nd and 4th quarters and turn them to 8ths.
- Select both newly created 8ths notes and using Option + Right Arrow move both of them one 8th forward.
While both selected notes do move as expected, the note on the 3rd beat disappears.
Here’s the snapshot of such utterly simplified example.