Three things:
A. Have you never let the battery die and just constantly left it charging, that can kill Lithium Ion Batteries
2. Have you tried disassembling it and moving around your flat wires? sometimes that fixes it, mine would randomly shut off due to pressure around the home button, but that was because the flat wire inside was sat awkwardly.
▲. Have you tried shutting your console off and on? sometimes it's a connectivity issue and not a battery issue.
A: I have never left them to die as I play pretty much 65% of the time in handheld which constantly charges joy cons.
2: Should i disassemble it? Im kind of nervous? Also its the left joy con not the right, it shouldn't have to do with the home button, I'm pretty sure the battery just cant hold its own unless constant charge flow
▲: Its a battery issue, it literally won't turn on the green lights when its off a charge flow. The console immediately says "battery depleted" and shows the left joy con logo. I had reset my switch like a thousand times.
@: So I found people with similar situations, literally all solutions were just "it started working out of nowhere!"