I am facing a very similar issue.

When including the placeholder (i.e. Dear {{ object.display_name }}, ... ) in the email text :

- It works perfectly for the test message sent to myself

- It doesn't work at all when sending to a list of contacts selected based on some criteria (reproducing the code and not the content)

I found a way to solve the issue.
I am creating an email template from the email template menu accessible in developer mode.
There I can switch the editor to see the actual code behind the template.
From the code, I can replace the placeholders by their QWeb content.

My example : this is the code to get the name of the object contact : 

" see comment up - as the t-out is processed in the answer ;-) "

There is a video that explains all this : 
https://www.youtube.com/watch?v=VQJodqSE5DM

Hope it helps you to get it done !