This is a bug or a undoc behaviour,
it is present on VFP6.5,7.1,8.1,9beta

Workaround is of course:

clear
set multilock on
?Cursorsetprop("Buffering",5,0) && returns .t.
create cursor abcd (abc C(200))
?cursorgetprop("buffering","abcd") && returns 1 ??  aspected 5
?Cursorsetprop("Buffering",cursorgetprop("buffering",0))
?cursorgetprop("buffering","abcd")
use dbf("abcd") in 0 again alias abcd1
?cursorgetprop("buffering","abcd1")
CLOSE TABLES all