Custom TODO explorer highlight in C# 6 interpolated string

I'm trying to create a custom TODO explorer tag to hunt for a particular substring in strings in a large solution.

Here are the settings I'm using:

If the substring is present in a "normal" string, it highlights fine, e.g:

var x = "fast:";

However, if its present in an C# 6 interpolated string, R# doesn't find it:

var x = $"fast:{DateTime.Now}";

I'm using Visual Studio 2017 Enterprise (26403.0) and R# 2017.1.20170407.124642.

Am I doing something wrong, or is this not supported currently?