I have some difficulty describing this properly (as can be inferred from the title), but here's how to reproduce the issue:

1. Copy some template code with prefilled parameter values, e.g.: {{Ping|a}}
2. Go to some Wikipedia page, e.g. https://de.wikipedia.org/w/index.php?title=Testpage&veaction=edit
3. Paste the code

On first sight, VE seems to do just fine adding that template. However, it is impossible to properly work with the inserted bit. You cannot, for instance, add some text immediately before or after the template (so as to, say, make it part of a sentence) -- you can only add new paragraph above/below it. Likewise, it's not possible to make it part of a list. This behavior differs from what happens when you use VE to generate the template in first place. (Chrome 55)