I opened case… Left half of backlit keyboard stopped working…How can I fix this issue?
You need to replace the keyboard assembly.
It might just be coincidence but the LED backlight circuitry on that side of the keyboard has failed. Remember this is a wear part and depending o on the pressure of the air, you may have damaged something.
Given the labor involved to disassemble a keyboard (the 2020 models and newer use rivets and not screws so they’re impossible to service) it doesn’t make sense to try and fix this.
I have very similar Macbook which came from service and it has same issue (also left side is not working)
You shouldn’t accept repair work that comes back damaged. At a minimum you should get a discount on the repair you took it in for.
Bottom line is you unfortunately have to live with this. There’s no setting or tweak that you can make that will fix a physical issue