Macbook Pro randomly sleeps/hibernates

My 2016 Macbook Pro has been entering sleep (hibernation?) without warning; I could be in the middle of typing, editing a photo, clicking/scrolling around etc., and the screen just goes black. Unlike with normal sleep (i.e. via the menu bar, the power button, or closing the lid), pressing any key doesn’t seem to wake the mac. I have to press the power button to wake (as if my mac was shut down), and then I’m brought to the enter password screen. 

The battery was replaced recently by a 3rd party repairer (I tried to get it replaced by Apple but they told me they cannot service it because my Macbook is considered vintage). I think this issue started after the battery replacement.

Strangely this has only happened while my Macbook was plugged in/on adapter power. I mostly use it plugged in though, I’ve been running it on battery the past few days to test if the random sleeps/shutdowns occur while on battery power—so far no. Today I just tried running it on adapter again; the sudden sleep/shutdown occurred within maybe 15 mins of use, as before.

Prior to the battery replacement I also had random sleeps/shutdowns—but only on battery power. I assume it must’ve been due to my 6y old dying battery (Mac OS battery service warning had gone off, and coconutBattery said the battery health was terrible).

Also, I installed Windows 10 recently on the mac via Boot Camp, and the random sleeps also happened a couple of times while I was using Windows. However with Windows I could wake by just pressing any key instead of having to press the power button. I went into Windows Event Viewer, and it said the reason for sleep was ‘power button or lid’ (I certainly was not pressing the power button or closing the lid accidentally).

So I went into Windows Control Panel—>’Choose what the power button/closing the lid does’, and set it to to ‘Do nothing’. It hasn’t happened in Windows since. However I’ve set it back to the default settings (‘Sleep’ instead of ‘Do nothing’) to test, and the random sleeps hasn’t happened again.

Just very strange to me. Maybe the 3rd party repairer just screwed up my mac? Any help would be appreciated.