Old 3DS XL shuts off when closed (no popping noise)

So i've got a 3DS XL here and it's acting a bit weird. Mostly works fine, controls, screens, audio, everything except for both cameras which are borked and also whenever i close it as it then almost immediately shuts off. I looked around online for what could be the issue and i'm still not sure what it is. Haven't ever messed with the insides, system wasn't dropped once to my knowledge, and it charges just fine.

I can play on it just fine too and it only turns off whenever i close it and i have to wait a while (with it opened) before i can turn it on again. It did turn off randomly this one time when the battery was nearly drained (17.5%) but i was able to turn it back on as it still had some charge.

The consistent part is that i never once heard a popping noise coming out from it (From what i've read, a popping noise means that there might be an issue with the ribbon cables). This actually used to be a relative's 3DS and they have an unfortunate habit of leaving things powered on and/or charging overnight and then forgetting about it, so it could be the battery? I actually tried reseating the battery but that didn't solve it.

Edit: I found out that it only turns off when the sleep mode mechanism itself is activated, as it did stay on while closed during a crash report where it couldn't go into sleep mode. (Nothing serious there, the SD card's a bit iffy).

Edit 2: Ok, so it's really not the mechanism as the GBA VC sleep mode (where it just dims the screens) will not shut the system off.