Here is an excerpt from my config file:

Type Last Edit = datetime
Preset Last Edit =$entry time
Locked Attributes = Last Edit
Subst on edit Last Edit = $date
Preset on Duplicate Last Edit = $date

I have another attribute called Part that I've made extendable.

When I duplicate an entry, Last Edit is updated with the current date correctly. However, as soon as I click the Add Part button next to my extendable Part attribute, and the page reloads to show the entry box for the Part field, the Last Entry field is replaced with a "-".

I have to submit and then re-edit the entry to get Last Edit to have a valid value again.

 *EDIT*:

I noticed that any time the page reloads while in the entry screen this happens, e.g. by selecting plain instead of html format.