I have an Alienware m15 R5 Ryzen Edition. 

Normal Internet, Netflix, it doesn't matter if you watch movies, etc. The battery status will be maintained at 100%.

When playing games that use GPUs (Lost Arc, Total War, Cyberpunk), the AC adapter is connected, but the battery charge gradually decreases.

About 2-3% every 10 minutes. 

Is the dedicated power adapter connected to your Alienware 15 unable to handle the power consumption?

I tried changing the adapter setting in BIOS to Primarily AC, Standard, and Adaptive.

Dell Power Manager User's Guide, Battery Settings