Upstream has released a new version marked as a critical bugfix, see here: https://community.ardour.org/node/8457 Reproducible: Always All that was needed install the new version was to rename the ebuild from ardour-3.5.380.ebuild to ardour-3.5.403.ebuild and recalculate the manifest. Everything worked as normal.
Thanks Koreth! I bumped the version, but it looks like it ignores the sse flag somehow nowadays and fails to build if march is set to something that doesn't support sse! I now ask gcc if sse is disabled and die if not with a useful message.... :-) Cheers!