Updating an Event doesn't trigger

We're using some plugin code to rewrite links in RTE content (from our GitHub repo to where they're surfaced on our site).

The code is triggered via the HtmlBeforeCreateEventHandler / HtmlBeforeUpdateEventHandler and works fine for discussions, but we've noticed it doesn't fire for (calendar) Events.

Here's the code;

using Telligent.Evolution.Extensibility.Version1;
using Telligent.Evolution.Extensibility.Api.Version1;
using Telligent.Evolution.Extensibility;

namespace Element14.AssetRepo
{
    public class RewriteGitHubLinks : IPlugin
    {
        public string Name => "zzz";

        public string Description => "ttt";

        public void Initialize()
        {
            Apis.Get<IHtml>().Events.BeforeCreate += new HtmlBeforeCreateEventHandler(this.Events_BeforeCreate);
            Apis.Get<IHtml>().Events.BeforeUpdate += new HtmlBeforeUpdateEventHandler(this.Events_BeforeUpdate);
        }

        private void Events_BeforeCreate(HtmlBeforeCreateEventArgs args)
        {
            // MY CODE
        }

        private void Events_BeforeUpdate(HtmlBeforeUpdateEventArgs args)
        {
            // MY CODE
        }
    }
}

I've put a breakpoint inside the Events_BeforeUpdate method and it never gets hit.

Any ideas why this isn't working?