To put a conclusion on this for others who might come after me — embarrassingly it was me — seems i had historically enabled support for shortcodes in my custom fields using:
// enables looking for shortcodes in all types of custom fields
add_filter('acf/format_value', 'do_shortcode');
There it was sitting in my functions.php but apparently this caused no trouble under php 7 and all the trouble under php 8.
So i have scoped this filter to the few fields where I know i need it eg:
// enables looking for shortcodes in all types of custom fields
add_filter('acf/format_value/name=fancy_footer_content', 'do_shortcode');
as clearly documented in https://www.advancedcustomfields.com/resources/acf-format_value/
