I managed to track down the problem, my /Views/Partials/Grid/Editors/Rte.cshtml was as below

@model dynamic
@using Umbraco.Web.Composing
@using Umbraco.Web.Templates

@Html.Raw(TemplateUtilities.ParseInternalLinks(Model.value.ToString(), Current.UmbracoContext.UrlProvider))

I looked in the src repository on git hub and changed it to

@model dynamic
@using Umbraco.Web.Composing
@using Umbraco.Web.Templates
@{
    var value = TemplateUtilities.ParseInternalLinks(Model.value.ToString(), Current.UmbracoContext.UrlProvider);
    value = TemplateUtilities.ResolveUrlsFromTextString(value);
    value = TemplateUtilities.ResolveMediaFromTextString(value);
}
@Html.Raw(value)

This fixed the issue, but VS complains that TemplateUtilities is obsolete, so what should the view be for 8.6.3?