According to Apple's Note (1st paragraph's reference is convoluted, but makes the point)
Prepare for APFS in High Sierra, APFS is said to be the default format for internal SSD drives and one can not opt out of the conversion. ...So, it would seem this may be what's defeating your effort to clone your external, HFS+ HS back to your internal SSD.
APFS is the default, can't opt out of, format for High Sierra installed
directly onto internal SSDs, but HS on an internal SSD is not
required to be APFS at the moment, although that may (will probably?) change down the road.
I had no problems installing HS on my internal SSD as HFS+, my only critical problem was with its Keychain Access component.
