hi,
are you sure that DPTR1 has not been selected as current DPTR before you do setb acc.2? And so you do not keep DPTR0 but just re-load DPTR1 which is modifyed by write_spi_byte() then.
When you need to save DPTR by some reason either use PUSH/POP for DPH and DPL or TOGGLE select bit! In example above I suggest you to use xrl A,#4 (or direct XRL WMCON,#4 as Michael suggests) instead setb/clr acc.2

Good days!