MacBook Air randomly turns itself off unless plugged in

Hello there, I'm wondering if someone is able to help.

My MacBook Air (2018) has been turning itself off randomly, and refuses to turn on when the power button or keys are triggered, despite the battery showing more than half left. It acts like it is dead.

It only turns itself back on when it is plugged in, and low and behold, the percentage usually shows more than 50%, but is different each time... so now I have my laptop plugged in 24/7, so it doesn't turn off while I am in the middle of working, which helps, but isn't practical and can't be great for the battery.

Has anyone had this happen to them before / anyone know any fixes for this? I've already tried antivirus/reset the SMC/PRAM etc. Any help would be greatly appreciated, thanks.