I think I found another bug in xtable (or placetable).
The last table in the MWE is not
showing the first row (the header row).
Remove the split setting (\startxtable[split=yes]) from the tables
because the "split" keyword
(\placetable[split]) from the float is enough to beak your tables across
pages.
What happens with \startxtable[split=yes] is that ConTeXt changes the
number of lines
it reserves on the current page before the break which isn’t enough to
fit the first line
of the table together with the caption and the header.