Gerrit adds spurious blank lines to commit messages when staged

Example: PS21 of https://codereview.qt-project.org/c/qt/qtbase/+/257000

When a commit is staged in Gerrit, it shows a new patch-set, that's got its Reviewed-by headers added; after that, it has some blank lines (possibly one per restage, not sure), which it marks in red. (Whether because it thinks they're a bad thin or because it thinks they're removed - relative to what ? - is unclear.)
This is a visual wart that I wish it didn't inflict !