Donal are right in sense of the Intel MCS51
manual, I think.
If you have other experiences, put the
question to your MCU producer whether is
their MCU "read-modify–write" compatible
with Intel.

The Intel MCS51 compatible MCU manuals write
generally this:
The normal read instructions will read the pin
state without regard to the output data latch.
The only exception is the read-modify–write
category of instructions.
They are listed as follows.

 ANL  (logicalAND, e.g.,ANL PI, A)
 ORL  (logicalOR, e.g.,ORL P2, A)
 XRL  (logicalEXIOR,e.g.,XRL P3, A)
 JBC  (jump if bit = 1 and clear bit, 
       e.g.,JBC P1.1,LABEL)
 CPL  (complementbit, e.g., CPL P3.0)
 INC  (increment,e.g.,INC P2)
 DEC  (decrement,e.g.,DEC P2)
 DJNZ (decrernent and jump if not zero, 
       e.g., DJNZ P3, LABEL)
 MOV  PX.Y,C (movecarry bit to bit Y of Port X)
 CLR  PX.Y    (clear bit Y of Port X)
 SETB PX.Y   (set bit Y of Port X)

Vaclav