$tu rnd timer slot not working correctly?

So the Timer Utilities rnd function is supposed to work like $tu(rnd, 5, 1, 1000)$ where it chooses a random number between 1 and 1000 every 5 minutes, right? How I'm trying to use it, no matter what I put in the timer slot, it runs every few seconds.

The code I'm currently using is in the background bitmap field, as follows:

https://picsum.photos/1080/1920/?image=$tu(rnd, 10, 0, 1084)$

There are 1085 images (0-1084) in the 1080/1920 directory at picsum.photos, and they are each numbered so they can be selected like https://picsum.photos/1080/1920/?image=548 for example. The above code does work in grabbing and displaying the images randomly, but it does so every few seconds, rather than every 10 minutes. Am I doing something wrong, or is there a bug or quirk that won't allow this to work?