[plt-scheme] Weird timeouts in web-server

With initial-connection-timeout set to 300 (5 minutes) I'm still getting
some deaths due to timeouts, and am at a loss.  I've hunted through
server code to try to figure out what is going on, but not with much
success.  Here is what is happening: I navigate to a page with a form.
 I click on submit for that form and it's intercepted by JavaScript to
send off to the server in the background a la AJAX and displays the
results when they return.  Now, I would expect that this background
submission has the full 5 minutes to complete (and if the time is
exceeded, it seems from my tests with a 30-second timeout to retry the
request once with the new connection and full timeout: if you could
point me to where the logic that does this is I'd be grateful).
However, in practice it is at times ended much more quickly (54 and 27
seconds seem the norm) and continues to die to timeouts in the same
timespan when resubmitted (which seems to me to indicate the time is
captured in the k-url of the form's action in some way, except that
the kill-time is stored as an absolute not an offset).

(It shouldn't matter, but we are using our own LRU continuation
manager that only differs in that not at maximum memory continuations
only live for 10 minutes and at maximum 2 minutes.)

Any insights are appreciated,
~NT
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