If you have a simple piece of custom middleware installed on the same website as SEO Checker, the presence of SEO Checker will cancel the execution of that middleware.

If you add a custom piece of middleware like this, to the app.UseUmbraco().WithMiddleware(...) part of Startup.cs, the middleware will not be executed when visiting a frontend page of the website. It will however be executed when visiting the backoffice.

class TrackingContentMiddleware
    {
        private readonly RequestDelegate _next;

        public TrackingContentMiddleware(RequestDelegate next)
        {
            _next = next;
        }

        public async Task InvokeAsync(HttpContext context)
        {
            IUmbracoContextFactory umbracoContextFactory = context.RequestServices.GetRequiredService<IUmbracoContextFactory>();

            using (UmbracoContextReference umbracoContextReference = umbracoContextFactory.EnsureUmbracoContext())
            {
                IPublishedContent? content = umbracoContextReference.UmbracoContext?.PublishedRequest?.PublishedContent;
                //Add tracking stuff here

            }

            await _next.Invoke(context);
        }
    }

If you then go ahead and remove SEO Checker from the website, the middleware will be executed on every frontend page visit.

Turning on/off rewrites and/or redirects in SEO Checker does not make a difference.

Moving the middleware outside of the app.UseUmbraco().WithMiddleware(...) will make it execute, but that is not where it's supposed to be, and won't make the middleware work as expected.

Note: The middleware in this post is an example.

Solution is running: Umbraco CMS 10.4.2 and SEO Checker 10.0.4.