When Mode->Unit display-> Enable Denominator Prefixes ----- is on.
I really don't think the Stefan–Boltzmann constant comes out right.
Try input : 5.6704W⋅(10^−8)/(s⋅m^2⋅K^4) and convert the output to Watts
With "fixes" off: 56.704(nW ∕ (K^4 ⋅ m^2 ⋅ s))
"fixes" on : 56.704(kW ∕ (kK^4 ⋅ m^2 ⋅ s))
??
nW = 10⁻⁹ W
kW = 10³ W
kK⁴ = 10¹² K⁴
kW / kK⁴ = 10³ W / 10¹² K⁴ = 10⁻⁹ W/K⁴ = 1 nW / K⁴
The answer you got with denominator prefixes on was thus correct.