Something very strange with the Customizer. When I hit customize from Frontend to customize specific post, got a weird autosave message, and url had ?autosave attached. Once I removed that I saw my changes on frontend.
WP autosave can be a little bizarre on occasion.
Worth checking that when making Customizer changes that you are viewing the published URL and not ?preview or ?autosave before hitting customize.
I also notice with WP 5 – the Save as Draft option in the Customizer randomly kicks in. Which can be very frustrating.