Steps to reproduce:

• import attached yaml, it will create 2 templates, where Parent inherits Child one,
• go to LLD rules for "parent_template_DEMO",
• open trigger prototypes for rule "child-lld-rule", which is inherited from "child_template_DEMO" template,
• create new trig proto and for expression select available item proto:
  
• finish its creation to have now 2 trig protos:
  

Try to export both templates together. Have a looks and see that the trigger prototype which we have created is not included to the export.
i.e. resulted YAML is the same as we used to import it.