Hello Tobia Scapin,
I hope you are doing well

For an on-premise Odoo 19 Enterprise installation, the best approach is to create a custom module that inherits the template:
Fix:

<template id="template_followup_report_hide_pending"                                                                                                                                        
            inherit_id="account_followup.template_followup_report">                                                                                                                           
      <xpath expr="//div[hasclass('o_account_reports_header')]/div[hasclass('print_only')]" position="attributes">                                                                            
          <attribute name="t-if">context.get("mail") != True</attribute>                                                                                                                      
      </xpath>                                                                                                                                                                                
  </template>     

I hope this information helps you

Thanks & Regards,
Kunjan Patel