fpprintprec with bigfloats

Using fpprintprec inside a block with
a bigfloat produces
results I don't understand.
------------------------------------------------------------
(%i1) fpprintprec : 8$

(%i2) bf1 : bfloat ( integrate ( exp(x), x, -1, 1 ) ), fpprec:45;
(%o2)                             2.3504023b0

(%i3) bfprint (bf, fpp) :=
  block( [fpprintprec ],
        fpprintprec : fpp,
        print ( " ", x, "  ", bf ) )$

(%i4) for x : 10 thru 18 do bfprint ( bf1, x)$
  10    2.350402387b0 
  11    2.3504023872b0 
  12    2.35040238728b0 
  13    2.350402387287b0 
  14    2.3504023872876b0 
  15    2.3504023872876b0 
  16    2.35040238728760291376476370119120163031143596b0 
  17    2.35040238728760291376476370119120163031143596b0 
  18    2.35040238728760291376476370119120163031143596b0 
(%i5) ( fpprintprec : 50, print ( bf1 ) )$
2.35040238728760291376476370119120163031143596b0 

--------------------------------------------
The number printed for fpp  >= 16 is the 45 digit number
originally generated with bfloat.

What am I missing here?

Ted Woollett