I’m revising this post because I discovered the culprit to the off-canvas nav not being visible on two pages. The HTML rendered, it just wouldn’t appear.
I had a hidden trigger for a modal and that conflicted with the off-canvas nav in the header.
I’ve swapped the modal for links to a page to solve the issue. I’m still curious to know whether this is a known issue? I recall not being able to have two modals on a page but wouldn’t have connected the dots to the off-canvas nav in my header. Now I’m curious if I’ve inadvertently made this same error on other sites.