How often do Artist Radio Playlists refresh? Mine seem to do so every day - can this be controlled?

[D3e]

Plan

Premium

Country

 Canada

Device

iPhone 6

Operating System

iOS 12.1.3

My Question or Issue

I use Spotify only in offline mode, downloading only on WiFi.  I have several artist radio playlists, which show that they are fully downloaded when I'm on WiFi; however, the next day, when I go to play them in my car, only a few of the songs are playable, and I get the "Downloading..." message (although it can't download at that point, because I have cellular data turned off).  I like having Spotify pick music based on an artist I like, but it's no good to me if I have to remember to open up the app (and leave it open) every day before leaving for work to ensure that all is downloaded. 

Is this a bug, or are these playlists designed to update every day (unlike, for example, the Discover Weekly, which downloads only once a week, on the same day of the week)?