Strange they should argue over something there's a clear WT ruling on.
It's clear but a little vague at the same time. It's the Watchtower way!
From freddo's post:
Whether a disfellowshipped person sits next to a relative or next to any other member of the congregation should not be a cause for concern as long as he behaves properly.
If all present, including faithful relatives, are endeavoring to respect Bible principles relating to disfellowshipping, and it is not becoming a cause for stumbling to the brothers
Assuming that all the elders were aware of the quoted article there could have been disagreement over whether the DF'd person was behaving properly, whether the relatives were "endeavoring to respect Bible principles" regarding DF'd people (watchtower speak for no undue association, I suppose), or whether it was stumbling others.
There is plenty of room there for interpretation and disagreement.