I recently had my MacBook (Mid 2015, Retina) repaired for hardware issues related to SMC failure. After about a day of it working, it started having the exact same issues, which is odd considering they replaced the entire logic board (as well as a few other components like the fans, magsafe input, even the auxiliary jack). I've since taken the laptop back.
In between having the MacBook in the shop I've been using my brother's MacBook (exact same model) while trying to eliminate any possible sources of the issues on my end. I'm guessing it was just a faulty repair on Apple's end, but I want to be safe. One thing we identified was a slightly damaged charger cable (exposed wiring at the end), which we've stopped using. However, considering that the same issue occurred soon after getting my MacBook back, if it is something on my end, it's not the damaged charger cable (at least not the second time).
The only thing I can think of is the magsafe cable that connects to my MacBook from my 27" Thunderbolt display. Is there any way to diagnose if this could be the issue? The magsafe adapter shows no discoloration, the LED indicator is accurate, and there's no visible damage to the cable itself. It also does actually charge any MacBook connected to it.
There have been spills on my desk before, but the water wouldn't have touched my MacBook (it always rests on a laptop stand)—however, I can imagine it might've touched the display's magsafe adapter while it was disconnected from the laptop. Again though, no signs of rust, etc. I really doubt it's the issue, but I want to be positive.