It looks like the package.json of a generated theme for 7.3 is coming in with outdated deps for _styled and _unstyled themes. The _unstyled theme in particular being so outdated is leading to missing lexicon icons, both the SVG files and missing icons from icons.svg.
Found when looking for missing icon from custom theme:
LPS-118583 "Change desktop viewport button icon"
Should be higher than 5.0.0