I disabled the spacing add-on which broke the layout even more.
Also noticed the the total width of the sidebars and content area combined is less wide than the navbars.
The total width should be 1380px however, according to an onscreen ruler the total combined width is 1300px.
FYI, recent Customizer and database settings were backed up prior to finding this issue however, it’s impossible to know when the problem might have first occured.
I’ve got database backups going back to near the beginning of the build if they are eventually required.