There was a deliberate change to the behavior of buildWithParameters in Jenkins 1.561, see linked issue and changelog.

What exactly is the problem here? Could you provide steps to reproduce the issue, what the expected result should be, and what the actual result is? Right now, it's unclear whether this is due to the deliberate change or some unexpected side effect you're reporting.

No response to comment asking for clarification in half a year, so resolving as Incomplete.

Sorry for the late response. I didn't notice the previous comment.

In my use-case, I have a web form with action=.../buildWithParameters target=_blank and method=post. I expect that when the form is submitted and the new tab is opened, that it won't be blank, but will be redirected to either the newly created job or if that's not possible (because the job hasn't started yet) to the project page.

/buildWithParameters is the programmatic API endpoint. It sends a Location header value specifying the location of the queue item with information about the queued job. So the behavior you're seeing is intentional.

You may be able to make /build work.

2 things to mention here :-

1) I'm sure the /buildWithParameters used to work and successfully redirected to the job page, so useful behaviour was lost (I get asked for it back every week or so)

2) The patch in the linked issue seems to try and include a redirect, so is it trying to do what this issue asks for and failing, or am I reading it wrong?