Thanks for pinging me, Xander Bakker‌!  No, I haven't seen this before and after checking, I don't see any other reports of this.

Jimmy Knowles‌ How are you!?  

I'm going to run this by a product engineer on the Pro team, but unfortunately it looks like a bug to me.  Basically, whenever I use Arcade to calculate null into a field (regardless of data type) it says that it succeeds and writes 0 to the field.

I tested with Text, Short and Double.  So that is a bit different from what Xander saw when running the calculation in ArcGIS Online.  I think we'll need to get a bug logged for this and Jimmy, I can get you attached to that when we have it.  Glad that you have an easy Python workaround. I'll follow up on this soon.

Cheers