"Delete missing" checkbox does not work when import template without checking "Update exising"

Steps to reproduce:

1. Export a template, then open with a text editor and make some change with a trigger expression like below:

   For instance: 
   modify from {HOST.item.last()}>1 to {HOST.item.last()}>0 

2. Import this changed template to the original environment with option like Screen Shot 2019-07-08 at 11.01.58.png
3. You will find the modified trigger expression[>0] was added and the old expression[>1] was also remained.

Result:
See Screen Shot 2019-07-08 at 11.21.58.png

Expected:
See Screen Shot 2019-07-08 at 11.22.50.png

When you import the changed template with option like Screen Shot 2019-07-08 at 11.06.32.png, only the new trigger expression[>0] was added.The most concern is the different behavior between checking and not checking "Update exising" option when import a template using "Delete missing" option.