Buck converter quiescent current is too high
Read spec again (2)
That is the non-switching current spec. 120 µA
What are your design specs?  {must haves &amp; nice to haves} idle, load , sleep
I'm not sure how FPWM mode is controlled, but that would explain your idle efficiency.
12V FPWM Graph below shows 5% eff @ 1mA meaning it takes 20mA to supply 1mA