Hello. Sorry for the late reply. I was focused on my midterm exam.
Anyway, I found out what the problem was. It's my sd card all along. When I format my sd card as an internal storage the phone prompts me with a message saying that it's slow and might affect my apps (which is weird because I can use it perfectly fine on my previous phone). It also just ejects itself either I format it as an internal or portable memory, without any warning message. That is the time when my power problems begin. So I decided to take it out, and that fixed it. I can now double tap to power on seamlessly!
(Exept when I just picked my phone from my jeans' pocket. For some reason, I don't know why, the double tap to power on doesn't work on this exact moment.)
Also, I haven't tried inserting a new sd card to try if it was really my old sd card's fault. I'll do that when I get some money.
Sorry for the late reply again. Cheers!