Left hand upper staff bug

Hello everyone.
As the title says, when I bring to the upper staff quaver notes that I want to be played with the left hand, the line connecting them ( Sorry, don't know the english word ) disappears. I'll attach a picture so you can see it.
I know I could avoid this by using the second voice in the upper staff, but I wanted to point this up anyway...
Do you know what could be happening?

Thank you!