To make weird lines appear on the right log, expand and shrink the left log by clicking it twice, then switch the right log to any non-l mode and then to l-mode.
Loglines retain these weird lines, but new loglines don't have them.
Expanding or shrinking the left log gets rid of the weird lines in the right log.
Thank you for the detailed explanation! I've seen this happen before, a case of the UI rendering being cut short so the lines don't disappear. I'll look into refining the scripts to make sure that can't happen.