Looks like an apt update from a few days ago killed Sway. 
Errors out with missing config files / display issues (kHz mismatch).
Tried reform-display-config single and rebooted but that unfortunately did not resolve the issue.
UPDATE
@mntmm provided the fix - https://community.mnt.re/t/psa-dont-apt-upgrade-libffi8-package-currently-breaks-wayland