Resolved: issue with SEO Checker package. If you have the SEO Checker package installed, you must "Enable redirect module". Otherwise it will brick the built-in url redirect handling. If you are using a custom lastChanceContentFinder, it will conflict with SEO checker - disable it (remove SetContentLastChanceFinder call) OR you can create a composer that fires AFTER SEOCheckerComposer and add code in your LastChanceContentFinder to check if the RedirectContentFinder found anything.

composer: using Umbraco.Cms.Core.Composing; using Umbraco.Cms.Core.DependencyInjection; using Umbraco.Extensions; namespace Core.Composers { [ComposeAfter(typeof(SEOCheckerComposer))] public class My404Composer : IComposer { public void Compose(IUmbracoBuilder builder) { builder.ContentFinders().Append(); builder.SetContentLastChanceFinder(); } } }

LastChanceContentFinder: public async Task TryFindContent(IPublishedRequestBuilder contentRequest) { //must set this to allow URL Redirect Management to work if (await _redirectContentFinder.TryFindContent(contentRequest) == true) { return true; } ...