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So he was already widowed in 1824. And would Martha be over 21 in 1824?

My understanding is that they are only valid for three months from complete reading. I have several odd cases in my tree.

1. banns were fully read in one parish where both resided, then read again starting almost immediately in another parish a long way away (again both said to be resident there), followed by marriage in the second parish. The groom was a soldier and it looked like he was posted suddenly while the first set where being read. The bride may have been the daughter of another soldier, but not sure.
2. Banns read in 1830s, but no marriage until 1860s. On censuses between these dates, couple lived together producing three children. He said he was married, she said she was his single "housekeeper". So I suspect someone objected to the original banns because they knew he was already married.
3 Banns read twice about 18 months apart in two different parishes, couple married straight after 2nd set. I think bride was under age when first set read, didn't do any further research as they turned out not to be related just a co-incidence of names.

If you don't find a second set of Banns or a licence corresponding to the actual marriage, it is possible that the vicar was ignorant of the fact that Banns had a "use by" date

Possibles off FindMyPast

19.10.1830 at Lambston Pembrokeshire William John to Ann Griffiths
25.1.1835 at St Andrews Cardiff William John of St Andrews to Anne Griffiths of Leckwith
18.9.1847 St Issells Pembrokeshire William John of Kingsmoor collier father James John collier to Anne Griffiths of Hill father James Griffiths servant on of the witnesses is an Isaac Griifiths

Willow x

The couple would be too young for the 1830 or 1837 marriages

They are William John b1815-1820 of St Dogmells, Pembrokeshire and Ann(e) Griffiths b1823-1826 of either of Tremain or Blaenporth, Cardiganshire. They appear to have had their first child in 1846 so should be married by then but I just cant find it.

But a marriage in St Issells - quite a long way from St  Dogmells, Tremain or Blaenporth - might fit if the couple were living in St Dogmells as a married couple. To marry without a scandal they would likely go to somewhere a good few miles away.

I have a match with 42cM across 3 segments. She has the same uncommon surname as my mother. She has been correctly identified by Ancestry as having common ancestors with me (our 2 x great grandparents). She is definitely my 3rd cousin on my mother's side. Our trees on Ancestry overlap. Our four shared matches have all been identified correctly by Ancestry as being maternal.  But Ancestry designates her as  "unassigned". How does the algorithm work that one out?