Thomas's father was yet another Thomas so easy to get things muddled.
I wondered if that might be behind my issue, since my great grandfather has the same name as his father except for the addition of a 2nd middle name.
It's odd, because if I look separately at my ThruLines list of ancestors everyone is showing from my tree as they should be. And when I look at the DNA matches where my great grandfather is our common ancestor, and open up the match details page, it shows him as our common ancestor, but then when I click to 'view relationship' it has replaced my great grandfather with my half 2nd cousin. I notice, however, that she is not shown in a box like all the others, which may be the way Ancestry indicates they have substituted someone else for the ancestor in my tree, presumably because they think I've made a mistake.
Now I'm thinking my 2nd cousin must have linked her results to the wrong person in her tree, and that is what's causing the issue, with the similarity in the names of my great grandfather and his father compounding the problem. Unfortunately I can't ask her as she died last year, and I don't think anyone is actively managing her results and tree at present.
However, as I said earlier, nothing has changed recently, and if my cousin is attached to the wrong person in her tree it's been like that for some years, so it must be some recent change in the way Ancestry is doing things. But it would explain why my cousin is now showing in the 'view relationship' page for her son as both his mother and great great grandmother, and on her brother's page as his great grandmother. (Echoes of 'I am my own Grandpa'!)
Question now is what can I do to correct this, particularly if I can't get in touch with anyone managing my cousin's account, and why the recent change? I don't know if, in the circumstances, Ancestry will be able to intervene to correct this or if I'm stuck with this muddle.