Your php setting for max_input_vars is 3000
Look at the field group you’re creating. Each field you’re creating has many inputs there may even be a bunch of hidden inputs. If you add them all up they are most probably near your 3000 limit.
When you submit the page to save your fields, if there are more than 3000, php simply throws away the excess, so they are not submitted to the script that needs to save your field group.
Does your host allow you to increase that setting? I would suggest upping it to 5000 see what happens. If your host does not allow you do change that php setting then you’ll need to figure out a different way to accomplish what you want to do using less fields.
