Cause of shutdown when sleeping

What would cause this unit to shutdown after around 20 minutes when put to sleep? We haven't changed any battery or sleep mode settings and the battery is a good one. We can close the lid and open it after a few minutes and all is well, but when it is left closed for say 20 minutes the unit is turned off. We tested it with our clean installed SSD and the same happens..... We have reset the SMC, PRAM, run diagnostics & there are no errors being reported, ..... baffled.