Error Steps:

• Log into the CMS Editor Interface.
• Select any Page and make an edit.
• Click Publish

The content will be published, but a validation error is returned with the following message:

Method not found: 'System.Collections.Generic.IEnumerable`1<!!0> EPiServer.Find.Helpers.ListExtensions.DistinctBy(System.Collections.Generic.IEnumerable`1<!!0>, System.Func`2<!!0,!!1>)'.

My Service Extension for setting up Content Delivery API is as follows:

namespace ClientName.Web.Infrastructure.ServiceExtensions;

using System;
using ClientName.Features.Common;
using EPiServer.Security;

using Microsoft.Extensions.DependencyInjection;

public static class ContentDeliveryApiServiceExtensions
{
    public static IServiceCollection AddOptimizelyDeliveryApi(this IServiceCollection serviceCollection)
    {
        // See https://docs.developers.optimizely.com/content-cloud/v1.5.0-content-delivery-api/docs/contentapi
        // This changes the root url from /api/episerver/v2.0/ to /content-api/
        serviceCollection.AddContentDeliveryApi(options =>
                         {
                             options.BaseRoute = "/content-api/";
                             options.RequiredRole = EveryoneRole.RoleName;
                             options.CorsPolicyName = ClientNameConstants.ExternalCorsPolicy;
                         });

        // See https://docs.developers.optimizely.com/content-cloud/v1.5.0-content-delivery-api/docs/contentapisearch
        // This changes the root url from /api/episerver/v2.0/ to /search-api/
        serviceCollection.AddContentSearchApi(options =>
        {
            options.BaseRoute = "/search-api/";
            options.SearchCacheDuration = TimeSpan.FromMinutes(5);
            options.RequiredRole = EveryoneRole.RoleName;
            options.MaximumSearchResults = 100;
        });

        return serviceCollection;
    }
}

Steps attempted to resolve the Issue:

• Checked out the commit directly before the packages were installed and the bug was no longer present.
• Checked out the commit where the packages were installed and configured as above and the bug was present
• Tried removing CORS, Bug still manifested.
• Removed both EPiServer.ContentDeliveryApi.Cms and EPiServer.ContentDeliveryApi.Search and commented out the code. The bug was not present.
• Reinstated just EPiServer.ContentDeliveryApi.Cms. The bug was not present.
• Reinstated both EPiServer.ContentDeliveryApi.Cms and EPiServer.ContentDeliveryApi.Search. The bug was present.

For now I've removed EPiServer.ContentDeliveryApi.Search from my client's project, but retained EPiServer.ContentDeliveryApi.Cms.  It's not a deal breaker for us to keep the Content Delivery Search API out of the solution as we would also include our own Search APIs tailored to the client's needs.  I would add that while the EPiServer.ContentDeliveryApi.Search package was installed, it otherwise appeared to function as expected and it is just the publishing of content that we observed as causing us an issue.

For reference, here are the other packages that were installed:

EPiServer.CloudPlatform.Cms @ 1.0.5
EPiServer.CMS @ 12.10.0
EPiServer.ContentDeliveryApi.Cms @ 3.5.0
EPiServer.ContentDeliveryApi.Search @ 3.5.0
EPiServer.Find.Cms @ 14.1.0
EPiServer.Labs.BlockEnhancements @ 1.1.1
Geta.NotFoundHandler.Optimizely @ 3.0.1
Geta.Optimizely.Categories @ 1.0.0
Geta.Optimizely.ContentTypeIcons @ 2.0.1
Geta.Optimizely.Sitemaps @ 2.0.0
MediatR @ 10.0.1
MediatR.Extensions.Microsoft.DependencyInjection @ 10.0.1
Stott.Security.Optimizely @ 0.2.0-beta
Swashbuckle.AspNetCore @ 6.4.0
Swashbuckle.AspNetCore.Annotations @ 6.4.0