@brett-haralson thank you, the support team has looked into it and emailed me.
@givemeawhisky thanks for the suggestion. But no, I did have a refresh there, and it’s working fine with the original site. As I was changing the button text in code to work around wix members lack of support for multi-language, the fact that multi-language wasn’t working meant the text didn’t get updated. Wix support answered that they found the language issue bug, but managed to work around it on this site. That did not resolve the permissions issue.
Edit: it turns out this was a database permissions mistake - the dynamic page had permissions, but the database was set as “form submission”, which evidently doesn’t allow loading dynamically post submission. Custom database permissions fixed this.