I'm a "Newby" at WikiTree,
Well you recently joined but you're giving good advice already.
I was convinced that the error was with the first source which was the only Finmypast primary source. I didn't look carefully enough at the Family Search ones which often have findmypast as secondary source. (Actually primary and secondary are the other way round, but you know what I mean).
My problem was I didn't look carefully enough, but your analysis of the problem was good and I am sure it will be useful to other wikitreers who are just finding their feet.