Formula User field is not showing in the ""Remind Whom"" section of Reminders.

I'm assuming you are using a formula-user field because often times a change on a different table, may affect the user in the formula.  If so, form rules would not work.

If they don't work in this case, try creating a report that filters by the current user, then set a subscription report to send out that report to the roles needed.

Just an alternative idea, but your use case (or other reading this) will dictate the route.