The latest beta (ConTeXt  ver: 2018.06.23 12:12 MKIV beta  fmt: 2018.6.23)
processes without a hitch but \seeindex is still not working as expected.

In

\starttext
\index{sample}
\index{fun+games}\index{fun+stuff}
%\index{fun}

\seeindex{fun}{sample} % now with heading (as it should be)
\seeindex{fun}{blah} % not shown
\seeindex{fun}{humbug} % not shown
\seeindex{fun}{bug} % not shown

\seeindex{fun+games}{entertainment} % not shown

\placeindex[n=1]
\stoptext

When I generate the indices for my book, \seeindex{A}{B} works only if it
is a unique x-ref and A is a heading. Multiple x-refs to the same heading
fail as do x-refs to subheadings.

Alan