You had recommended that I ensure the 404 page is in a hidden menu item off of root. I confirmed it already is.
I further explained that the redirection worked prior to SPPB 5.5.0, 5.5.1, and 5.5.2 so you would know that the issue only developed with SPPB 5.5.x, just as it did with so many others. The custom page redirect no longer works since you created SPPB 5.5.x, and this includes the lates SPPB 5.5.2.
This is the support forum to post issues when things don't work, correct? This is what I am doing. This is the purpose of my post. Per the post header above, "SPPB 5.5.2 Still Displays Wrong 404 Page." How can I help you understand the reason for my post in language any clearer than that?