I've just spent a while trying to work out what was wrong with my installation of Mosaico. I am getting random 1 pixel lines in the middle of emails and assumed I had done something wrong. I haven't as I've now found https://mosaico.io/email-client-tricks/outlook-2016-weird-1px-horizontal-lines/ and it turns out to be a bug in Outlook that Microsoft have done nothing about since 2016. Its fine with other email clients. I think I just have to live with it. It is not Mosaico specific.
Posting this hear a) to save other people's time b) in case anyone has come up with a workround (unlikely from other github etc posts on the subject.