Windows 10 Home 1809, Publisher 1.7.0.257.
The attached GIF shows what's happening better than I can explain it.
Basically, selecting some cells from the bottom table actually selects some cells in the upper table, sometimes.
Same document that I attached to this post: https://forum.affinity.serif.com/index.php?/topic/80965-spacing-between-table-cells-in-affinity-publisher/&tab=comments#comment-426146