Hi Gary-R,
Thanks for reaching out using Apple Support Communities. We're here to help.
We understand you're noticing a playlist is not updating changes on your iPhone.
Are you an Apple Music or iTunes Match subscriber? If so, you'll want to make sure Sync Library with Apple Music is enabled on both your iPhone and the Mac. The article below will assist you with this.
Use Sync Library to access your music library across your devices
How to turn on Sync Library on your Mac
1) Open the Apple Music app.
2) In the menu bar, choose Account and make sure that you're signed in with the same Apple ID that you use with your Apple Music subscription.
3) Choose File > Library > Organize Library. Then select Consolidate files.
4) Choose Music > Settings or Music > Preferences.
5) Click the General tab.
6) Select Sync Library.

7) Click Ok.
8) Wait for your music library to sync. To check the status, go to the bottom of the sidebar and click Updating Cloud Library.
9) Turn on Sync Library on your other devices.
How to turn on Sync Library on your iPhone or iPad
1) Make sure that your iPhone or iPad is signed in with the same Apple ID that you use with your Apple Music subscription.
2) Go to Settings.
3) Tap Music.
4) Turn on Sync Library.

5) Turn on Sync Library on your other devices.
After ensuring your Apple Music subscription is active, and sync library is enabled across devices, check the playlist again for updates. If not updated, check to see if there is a status message indicating that updates are pending. This may be towards the bottom of your Music app
It is also possible that some music may not be eligible to match and sync with iCloud Music Library. Examples of tracks that this could apply to would be tracks not purchased from iTunes, was purchased with a different Apple ID, or is too large. Try adding different types of songs to this playlist to see if the issue is only occurring with certain songs. The following article will help identify on your Mac which songs in your library are able to be synced.
If songs are missing from your music library after you turn on Sync Library
If the original problem persists after trying the previous steps, please provide us with specifics on what you see for more help, and we will continue from there.
Cheers!