Why does the QUAD Enable bit in the configuration register of my S25FL-S get constantly cleared to 0 when I use the WRR command?
The WRR command (Opcode: 01h) of the S25FL-S serial NOR Flash can accept either 8 or 16 data bits to be programmed. In other words, it can be used to program the Status Register alone (8-bit WRR command), or the Status Register and Configuration Register (CR) together (16-bit WRR command).
However, as soon as the user sets the QUAD bit CR  to 1, only the WRR command format with 16 data bits will be allowed. You should not write to the Status Register only (8-bit) using the WRR.
Not conforming to this rule may lead to inadvertent clearance of the QUAD bit, which in turn may lead to system crash or malfunction due to misconfiguration of the Flash Memory.