Hi Kellskyler,
Thank you for posting. I understand you're finding that music playback does not resume when you lock the iPhone and then unlock it. I'd be looking into this as well. I'm happy to see if I can assist.
Are you playing streaming content from Apple Music, or is it downloaded to your device? Do you notice a difference between either of those?
Have you tried to Restart your iPhone since this issue started?
If you haven't already, updating your device may help: How to back up your iPhone, iPad, and iPod touch and Update your iPhone, iPad, or iPod touch
If the issue persists after you complete the steps above, try deleting and reinstalling the Music app: Delete built-in Apple apps on your iOS 12, iOS 13, or iPadOS device or Apple Watch. Keep in mind that when you delete the app, any downloaded music will be removed, so you'll need to re-download it after.
Feel free to post the results here.
Take care.