USB ground on MSP430 Launchpad interfering with Comparator_A input voltage
The answer from discussion above:

"it was actually the internal pulldown despite the value not making sense. When I configured P1.5 as a comparator input, Grace greyed out the column to configure the pulldown so I figured it was not available when using the comparator alternate pin function. Turns out, if you go into the other tab to set up the registers manually (via checkboxes), the pulldown was still enabled in the register."