When charging, 2015 MacBook shuts down

Since installing Sierra, when its lid is closed and my 2015 MacBook is charging, it shuts down at random times for no apparent reason.

When it's not charging it doesn't shut down.

All software is fully up to date, the SMC and PRAM have both been reset, multiple times, and there is plenty of unused SSD space.

There is no shutdown or sleep scheduled.

Any ideas?