When utilizing the IList<string> property in CMS mode within a local block, the property's built-in UI will not function properly. Conversely, if the IList<string> property is directly employed on the Page, this problem will not arise.

This issue occurred starting from EPiServer.CMS 12.18.0. I tested it on the previous version and it worked fine.
Details: I created a local block below :

using System.ComponentModel.DataAnnotations;

namespace Alloy.Models.Blocks
{
    [SiteContentType(GUID = "937C7DB8-EF6C-4E7B-B128-63818EB9DA67")]
    [SiteImageUrl]
    public class TestBlock : SiteBlockData
    {
        [Display(
            GroupName = SystemTabNames.Content,
            Order = 1)]
        [CultureSpecific]
        public virtual IList<string> Tags { get; set; }
    }
}

And use it as a property on the page :

using System.ComponentModel.DataAnnotations;
using Alloy.Models.Blocks;

namespace Alloy.Models.Pages
{
    /// <summary>
    /// Used to present a single product
    /// </summary>
    [SiteContentType(
        GUID = "17583DCD-3C11-49DD-A66D-0DEF0DD601FC",
        GroupName = Globals.GroupNames.Products)]
    [SiteImageUrl(Globals.StaticGraphicsFolderPath + "page-type-thumbnail-product.png")]
    [AvailableContentTypes(
        Availability = Availability.Specific,
        IncludeOn = new[] { typeof(StartPage) })]
    public class ProductPage : StandardPage, IHasRelatedContent
    {
        [Required]
        [Display(Order = 305)]
        [UIHint(Globals.SiteUIHints.StringsCollection)]
        [CultureSpecific]
        public virtual IList<string> UniqueSellingPoints { get; set; }

        [Display(
            GroupName = SystemTabNames.Content,
            Order = 330)]
        [CultureSpecific]
        [AllowedTypes(new[] { typeof(IContentData) }, new[] { typeof(JumbotronBlock) })]
        public virtual ContentArea RelatedContentArea { get; set; }

        [Display(
            GroupName = SystemTabNames.Content,
            Order = 350)]
        public virtual TestBlock TestBlock { get; set; }
    }
}

Issue on CMS 12.18:


And it worked fine on CMS 12.17.1: