Can't manipulate glue added by tex.preparelinebreak

[Max Chernoff via ntg-context <ntg-context@ntg.nl>]

Hi Hans,

The new tex.preparelinebreak is great (thanks again), but I can't seem
to figure out how to manipulate the parfillskip nodes that it adds.

MWE:

\newbox\testbox

   \startluacode
       function test(head)
           if head.id ~= node.id "par" then
               return head
           end
   
           new_head = node.copylist(head)
           tex.preparelinebreak(new_head)
   
           local parfillskip = node.tail(new_head)
           inspect(parfillskip) -- Confirm that this really is a parfillskip
           parfillskip.stretchorder = 0
           parfillskip.stretch = 0
   
           local broken = tex.linebreak(new_head)
           tex.setbox("global", "testbox", broken)
   
           return head
       end
   \stopluacode
   
   \starttext
       {\dorecurse{209}{Hello }\par}\page
   
       {\parfillskip=0pt\dorecurse{209}{Hello }\par}\page
   
       {\ctxlua{nodes.tasks.appendaction("processors", "after", "test")}
        \dorecurse{209}{Hello }\par}\page
   
       \box\testbox
   \stoptext

I would expect that pages 1 and 3 would be identical, and that pages 2
and 4 would be identical. However, page 4 is the same as pages 1 and 3,
which isn't what I'd expect. I can do a similar idea in LuaTeX/MkIV and
get the expected results, so I'm not too sure what I'm doing wrong here.
I'm probably just missing something obvious, but I'm not too sure what.
Any ideas?

Thanks,
-- Max
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