A little bit more information was revealed after turning on DevErrors:
Not sure how this issue came about but I imagine it's likely only to occur in an upgraded environment. Away, after a bit of digging I came across the following article which seemed to describe the same problem (albeit for the lead form):
So I decided to export my contact entity and perform a search on all cases where "rowspan" has a value greater than 1 (e.g. rowspan="4"). I came across 6 cases and in each case I performed a count of the row elements and they matched up with the rowspan count. Until the very last one that is...
In short, the screenshot below is what I found.
And just as advertised this resolved the issue.