UITableView auto resizing row constraint breaking mysteriously on iPhone 6Plus

0.333 on a 3x scale display (which iPhone 6+ is) is probably connected with the cell separator.

Note that your constraints don't set up the size of the cell, they set up the size of the contentView. But the cell has to add 2 pixels (= 0.666 points) to the cell height for the cell separator. Autolayout tries to keep view positions on integer boundaries so adding 0.666points to the cell height can result in adding 0.333 to your content height.

You can avoid the error by setting your table separators to None. Although setting up one of the priorities to 999 (usually the bottom priority) as the other answer has suggested is a good solution in general.

This warning is telling you there's a conflict in your constraints. Reduce the priority of the height constraint to 999 and it will go away. Tested it in your Github project and worked perfectly.