No sound output using Scarlett 6i6 with mono tracks in Browser

I found an issue where I cannot hear clips which have either single or dual mono channels playing back in the Browser, but only while using a Scarlett 6i6 as the system sound output device. I can hear the same clip in the timeline, or when switched from dual mono to stereo, or when played through other system sound outputs. I can even hear the audio if I begin playing back a clip wile outputting to internal speakers, then switch output devices in System Preferences; however, as soon as I click back into FCPX, the audio is muted. The meters still show the audio levels, even though I cannot hear it. Other programs output via the Scarlett, even while FCPX is in playback.

Due to this specific behavior, I'm considering this to be a software glitch in FCPX, rather than a hardware or OS issue. Anyone else see something similar come up? In the meantime, I'm just skimming clips in the Browser with my internal speakers on.