Ah, I see. It looks like the date object is getting stringified when it passes through the trigger - in the local repro I tried, I had it set up to define the object in the query itself, like so:
Is there any way you can break out the lastUpdatedAt field to be defined in the query itself? By combining it with the doc object somehow? It seems like if you pass the new Date() object in the Firebase query instead of the JS query, it should work.