It's been a while, when i download something from play store or even update apps, downloading pauses few minutes after i lock the screen. ( I am using high speed Wifi )
I use APKPure to solve this problem.
This might be happening for a number of reasons (and a simple reboot might fix your issue).
1) Check if you have enabled Data Saver by going to Settings -> Network & internet -> Data Saver
2) Check if you have any battery optimization settings enabled by going to Settings -> Battery -> PowerMaster and/or Battery Optimization
3) Clear the cache and data of the Google Play app by going to Settings -> Apps & notifications -> Se all apps -> Google Play Store -> Storage and cache -> Tap clear cache -> Tap clear storage -> Reboot your device and open Google Play again
I excluded play store from battery optimization and it solved my problem.