Hello riddha,
Welcome to Apple Support Communities!
We understand the importance of having the music at Hi-Res Lossless when your iPhone screen is locked. We'll be glad to assist.
We'd like for you to try some steps that will delete any of your downloaded music from your library. This process is completed by going to Settings > [your name] > Media & Purchases. Tap Sign Out. Stay logged out during the next steps.
Turn off Sync Library under Settings > Music then try powering off your iPhone: Restart your iPhone Wait a few moments before restarting and then sign back in to your account. Turn on Sync Library again.
After testing, see how your music responds when your iPhone screen is locked. If the same issue continues, please reach out to Apple directly for further guidance: Get Support
Have a nice day.