This is a global issue with all tablets of this model running v6.3.9 OTA update, but since the model is relatively old, the company is not bothering to fix this by releasing a new OTA update.
There is no official way to downgrade. One way out is to hack the tablet and install a custom ROM from xda-developers site like some have done. But there is a risk of bricking the device that way.
Instead, I decided to raise a ticket with Asus but there is no action on that front. Hence, I decided to create a change.org petition requesting ASUSTek Corporation to fix this by releasing the next OTA update. If enough people sign my petition, there is a chance that ASUS might listen and release this update. Please help me by signing this petition:
https://www.change.org/p/asustek-please-fix-the-battery-drain-issue-in-your-latest-ota-update-v6-3-9-for-fonepad-me175cg?recruiter=337404987&utm_source=petitions_show_components_action_panel_wrapper&utm_medium=copylink