Charging is just fine, but the battery drains when I’m playing demanding games or performing resource-intensive tasks

Thunderbolt 5 cables support power delivery of up to 240 W. However, actual power delivery depends on the specifications of both the power source and the device receiving power.
For high-power devices such as gaming laptops and creator laptops, the battery level may gradually decrease even while charging if the power supplied is lower than the power consumed.
Use the original AC adapter supplied with the device, or check that the maximum output of the power source meets the device’s power requirements.