I’ve realised this issue is due to the “Development Store Preview” function:
Turning this off loads the current version instead.
It’s covered a bit in this persons thread: Clarification on Checkout UI Extension Visibility in Checkout Editor
@KyleG-Shopify - you mentioned something similar in that thread, but this seems to be about extensions rather than the settings you pointed out.
Can you offer any guidance or documentation on how this feature works and what it does?