I’ve setup test instance of WordPress multisite with domain mapping locally. But it did not repeat this issue. URE works as expected from any domain (primary or mapped) in URL.

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I will look on the site settings, make some tests on-line.

I suppose that moving the part of parameters responsible for the presentation, from the POST to GET (adding to URL) may resolve the issue. It depends if redirect takes place after POST data was really processed by plugin code. And it will not help if redirection occurs before POST data processing by URE plugin.

It’s not a quick fix and will require some time to refactor the code any way.