Peter, The SBUF is unique, the fact that:
1) it is 2 registers, one write, one read.
2) a write starts something that is not depending on a bit in the write.
This does make it possible/likely that some general sfr rules do not apply.
e. g. does a read-modify-write read the XSBUF, modify it and write to the RSBUF or is that a unique means of reading the XSBUF.

None of this is discussed in the data sheets, so who knows.

Erik