Prem. sub. - Note 9 using DATA instead of WIFI at home! Why??

[fgdfg]

Plan - Premium

Country - USA

Device - Samsung Note 9

Operating System - Android 8.1.0 (not rooted or any other funny biz)

My Question or Issue

I very recently discovered while playing some new music (music that wasn't downloaded to the phone or saved in any way) on my phone that my phone was using DATA instead of my home WIFI connection to play these songs. The wifi is certainly strong enough, I was a few feet from the router.

I always assumed that on smartphones the wifi would be always be prioritized over data when there's a stable wifi connection.  What exactly am I missing here? My PC certainly plays new music over the same wifi without any issues (because, obviously, it has no data plan). All other major apps switch to wifi when there's a stable wifi connection - email, texting apps, youtube ... etc. etc. etc. 

This has been asked before. 0 satisfactory or definitive answers. So, I'm actually DOUBLE paying (the service and the data) when there's a perfectly good home wifi connection that can be used?

Yes, I tried unistalling/reinstalling. No, I'm not going to turn on/off my data every time I want to listen to something new at home. No, I'm not lowering my sound quality, it's what I pay for. No, I'm not going fill up all my phone memory with DL'ed playlists. [just some of the suggestions in other threads].

I just want to know, is this how it's supposed to "work" or am I experiencing a bug? Not much point in a subscription if this isn't a bug, tbh - I can make playlists on youtube and get pretty much same results for free.