comparator isn't behaving as it should
It seems most likely that you have either tied the \$\rm\overline{CS}\$ input high, or you left it unconnected and it randomly drifted high due to static charge or capacitive coupling to nearby objects.

With \$\rm\overline{CS}\$ high, the output goes into a high-impedance state as described on p. 16 of the datasheet, which allows R3 to pull the output high.

Edit 

In comments you now confirmed you left it unconnected.

As a general rule, you should never leave CMOS inputs floating, as this can produce unpredictable circuit behavior (even oscillation and circuit self-destruction in certain worst case scenarios). If a chip has an internal pull-up or pull-down allowing a logic input to be left unconnected, it will nearly always be mentioned in the datasheet.