This is an extract of the console error from the updated site:
jQuery.Deferred exception: _ is not defined transform/options.ajax.processResults/items<@https://mydomain.com/wp-content/plugins/meta-box/js/select-advanced.js?ver=5.6.7:44:7
Can you please deactivate all plugins except Meta Box, MB AIO and switch to the standard theme of WordPress then recheck this issue? It could be a conflicting issue with another plugin like optimization or caching plugin.
It's a site made with oxygen builder and an underscore.me theme (oxygen disables themes). I have tried to disable all plugins except Metabox and Metabox AIO, I have also enabled the Twenty Nineteen theme.