MCP23017 not causing interrupts using a normally open switch on power up
1.7.5
 INTERRUPT CONDITIONS
There are two possible configurations that cause
interrupts (configured via INTCON):

1.
Pins configured for interrupt-on-pin change
will cause an interrupt to occur if a pin changes
to the opposite state. The default state is reset
after an interrupt occurs and after clearing the
interrupt condition (i.e., after reading GPIO or
INTCAP). For example, an interrupt occurs by
an input changing from ‘1’ to ‘0’. The new initial
state for the pin is a logic 0 after the interrupt is
cleared.

2.
Pins configured for interrupt-on-change from
register value will cause an interrupt to occur if
the corresponding input pin differs from the
register bit. The interrupt condition will remain as
long as the condition exists, regardless if the
INTCAP or GPIO is read.