Nexus 5X getting pretty warm and battery draining quickly when idle in TWRP

[BS7y]

After experiencing this before but couldn't exactly quantify the time and the battery drain I performed the following steps to reproduce the battery drain during the time the phone is being idle in TWRP with screen off:

1. Booted TWRP 3.0.0-1 from bootloader
2. TWRP showed battery percentage: 91% and CPU temperature: 24°C
3. Left the phone in idle state for 50 minutes (screen off after 30 seconds)
4. TWRP showed battery percentage: 79% and CPU temperature: 30°C

=> The phone felt unusually warm in the area of the fingerprint scanner upwards to the top of the phone.
=> 12% drain in 50 minutes = appr. 1% drain every 4 minutes
Normally, battery drain is appr. 2% overnight (appr. 7 hours) with activated airplane mode. So in TWRP the idle battery drain is appr. 50 times higher than this.