Virtual Item lastUpdate bug or design?

I’ve been working on some rules and have noticed that my items that aren’t linked to a channel, don’t seem to have a valid lastUpdate time.

my items are created using the ‘+’ icon in PaperUI->Configuration->Items and assigned to a group called gPlaylists

I’m using JDBC mysql as a persistence service with

Items {
    // persist all items once a day and on every change and restore them from the db at startup
    * : strategy = everyChange, restoreOnStartup

my script

rule "Play Playlist"
	Item gPlaylists received update
	val playlist = gPlaylists.members.sortBy[lastUpdate].last as SwitchItem

always fails as the lastUpdate value of all my items is null.

If I change the group to a group of door Contacts linked to contact Things, it works fine. Is the lastUpdate filled in by a binding so if there is no binding, there is no lastUpdate?

Strangely, I just ran a test script and some of the items in the group now have valid lastUpdate values, though in my testing I have updated all of them so I’m not sure what triggers the setting of that field

deleted items and then re-added them and now they are updated, not sure what the issue was