apple music mobile data use

Just to clarify some things:

1. Apple Music tracks need to validate an active subscription to play. While they can be played while offline, if its been too long since the last time it was connected to the internet, it may require an active connection to verify the status of the subscription again. It should not try to verify every time they are played, and as mentioned a quick connection over either cellular data or Wi-fi should be all it takes.
2. When you verify the data usage, were are you checking this? The Data usage shown in Settings->Cellular / Mobile, is calculated since the last time you manually have reset the usage. It is not calculated based on your plan and it's cut off or renewal date. So will continue to add data usage until you manually go to the bottom of that screen., and tap "Reset Statistics". If it says 40GB, it's likely adding up many many months since the last time you manually reset it.

link->Check the cellular data usage on your iPhone and iPad - Apple Support

link-> Add music to iPhone and listen offline - Apple Support

I'm experiencing part of your problems, and it started recently. (I've used Apple Music for years and it's a new problem for me.)

I'm usually keeping cellular data switched off for Apple Music, so I've not had problems with too much data usage. But the app has stopped playing songs from playlists that are marked as downloaded when I'm not connected to wifi. Today I noticed that I can switch cellular data on for just a couple of seconds when a playlist isn't working, and then switch it off again, and it fixes the problem. It seems almost like the app doesn't know what's downloaded on my device and what's not, unless it can talk to Apples cloud services. That's certainly not the way it's supposed to work!

As revey426 said "the amount of times i need to enable data to validate my subscription is just silly". If it is about validating subscription, or whatever the app needs to do using cellular data...

You obviously don't understand the problem. Even if I've paid for my subscription yesterday, like I've done monthly for many years now, Apple Music requires that I turn on cellular data every time I'm out for a walk, even the following days after paying for subscription and even after I've checked that the songs should be downloaded on my phone. It didn't use to work like that, it shouldn't work like that, and it's against Apples own words, saying that you can download playlists to your device and listen to them while offline. It's not possible form me to do that any more, but it used to work. There is definitely a bug involved. Can I explain it more clearly in case you still don't get it?

No. It seems you don't understand how online subscriptions work. The subscription status is stored on Apple servers since it needs to be accesible form multiple devices at any time. If it lived on your device, then If you cancelled the subscription from a different device, and it was embedded on another one, then you would have access to your songs without having to pay the subscription forever. That's akin to calling up your local gym and saying you have a valid subscription because you have it stored in a drawer at home. The gym holds your subscription on their system and verifies its status when you arrive there, you don't get to just show up and say its active and they must let you in.

As I also said, it should not require the subscription verification too often. There may be something wrong if it tries to verify very very often.

Spotify requires the verification too every so often for downloaded songs. If you leave Spotify too long without connecting to the internet it will say that it needs to connect to verify the subscription.

Spotify support forums: https://community.spotify.com/t5/Android/Premium-gt-Play-off-line-with-no-service/td-p/1306385

You should also be aware that if you are offline for more than 30 days you will no longer be able to listen to anything you have downloaded unless you go online again to verify your subscription status.

All music subscriptions do this every so often. It is an industry standard. Spotify does it every 30 days.

https://9to5mac.com/2015/06/09/apple-music-offline-listening/

In regards to the cellular stats in Settings->Cellular, As I said, unless you manually reset those stats, they don't get reset. There is no magic way for them to get reset unless you yourself reset them. If you did, then yes it would be counting form that point forward. The date of last reset is visible there too. If you have never manually reset the stats, then it will likely show the date the iPhone was first activated.

Beyond that, it's hard to be sure that it's trying to verify, since you have provided no information at all, also verification should not use a lot of data. Maybe a few kilobytes if that, to query they status of the subscription.