Unique filter does not seems to apply on concatenated list

I have two lists that I want to concatenate and then filter for unique values only. I figured this should be possible in Jinja2 using the unique filter, but, I'm finding it doesn't work the way I expected. I've used a few different live Jinja parsers to test and they all seem to end up with incorrect output.

For reference, the site I'm using to test my code is https://j2live.ttl255.com/

With an input of data that looks like this:

this:
  - one
  - one
that:
  - one
  - two

And a template that looks like this:

{{ this + that | unique | list }}

The rendered output is just the result of concatenating the two lists, when they clearly have duplicate values:

['one', 'one', 'one', 'two']

I've tried a bunch of different permutations here and they all end up the same way. Does anyone know how to get this to work as expected? Or am I missing something here? With the intent of the filter, I figured this would work.

I also want to point out, that the expected behaviour does occur when the lists are not concatenated. For example:

{{ this | unique | list }}

Results in:

['one']

The precedence is taken by the filter over the union of the two list.
What your code is effectively doing is:

{{ this + (that | unique | list) }}

So, what you really want to do is for the union to happen first, which you can achieve by using parenthesis:

{{ (this + that) | unique | list }}

Which results in your expected:

['one', 'two']

This is backed up by this comment on their issue tracker:

> Filters and tests have higher precedence than binary operators. For example {{ [3] + [2,1] | sort }} yields [3, 1, 2], not [1, 2, 3].

^{As commented by ngaya-ll}