The problem is the mapFieldByFieldName
is expecting to match a field with that name from the Sitecore item, not a custom computed field in your index so the field reader is never called.
I don't know how to achieve this from config, but if you do not want to directly use HAP but are willing to use some code then after you paste your fields together in your computed field class just do what Sitecore does in the GetPlainText()
method:
string input = "concatenated string";
return HttpUtility.HtmlDecode(Regex.Replace(input, "<[^>]*>", string.Empty));
or use the util method Sitecore.StringUtil.RemoveTags(text)