QuickBooks online sure is a piece of work!
I don't know why it does this. Seems overly rigid. Still, carrying lost inventory as an asset doesn't make sense to me: It's lost, so it's not worth anything.
I think inventory shrinkage should be an expense, and you should recognize it when you realize it's gone. That's rather standard, as I understand GAAP.
I suppose if you want to hide the fact that you're inventory is losing value you could create a Journal that decreases the Shrinkage account and puts the money somewhere else.