Image resizing on ip restricted dev site


I’ve been using CF image resizing for over a year on an IP restricted dev site (using .htaccess IP protection, not a CF WAF rule).

This did work fine up until a few days ago but resized images don’t now display (the original displays fine) and I get the following error code via the CF image resizing page:

Origin returned 4xx HTTP status code

If I remove IP restriction, the resized images display fine again.
I have tried allowing all the CF ip addresses but to no avail.

I’m struggling to understand why this would suddenly stop working?