Based on your description, I understand that when some users visit SharePoint site, site URL suddenly changes from http to https. If it is wrong, please correct me.
You should go to the central administration of production environment to check whether it is configured alternate access mappings.
Have you installed system update or CU patch in the production environment?
You could refer following articles to configure alternate access mappings correctly.
SharePoint 2013: Convert SharePoint web application from HTTP to HTTPS
SharePoint HTTP to HTTPS redirect
How to Change a SharePoint Web Application from HTTP to HTTPS?
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