Have noticed this before, so thought I would report it.
If you have a workflow actions, say “Create an account” linked to some email/password fields.
If you delete the action, and then add another action wanting to use the same fields, they are now missing in the list of values.
In the above example, the email/password field on the form don’t show up when you click into box for “Email”.
If you close the window and re-open…they are there.